Then you have multiple options:
Preaggregate using a derived table or CTE:

SELECT W.ID,
   W.Group,
   SUM( WD.Payment) AS TotalPayment,
   SUM( W.Invoice) AS TotalInvoice
FROM Widget W
JOIN (SELECT iWD.ID,
      SUM( iWD.Payment) AS Payment
   FROM WidgetDetail AS iWD
   GROUP BY iWD.ID) AS WD ON W.ID = WD.ID
GROUP BY W.[Group];

WITH AggregatedWidgetDetails AS(
  SELECT iWD.ID,
    SUM( iWD.Payment) AS Payment
  FROM WidgetDetail AS iWD
  GROUP BY iWD.ID
)
SELECT W.ID,
   W.Group,
   SUM( WD.Payment) AS TotalPayment,
   SUM( W.Invoice) AS TotalInvoice
FROM Widget W
JOIN AggregatedWidgetDetails AS WD ON W.ID = WD.ID
GROUP BY W.[Group];


Use CROSS APPLY (this is usually slower).

SELECT W.ID,
   W.Group,
   SUM( WD.Payment) AS TotalPayment,
   SUM( W.Invoice) AS TotalInvoice
FROM Widget W
CROSS APPLY (SELECT SUM( iWD.Payment) AS Payment
   FROM WidgetDetail AS iWD
   WHERE W.ID = iWD.ID) AS WD ON W.ID = WD.ID
GROUP BY W.[Group];


Or use a small hack that might have some problems with rounding.

SELECT W.Group,
   SUM( WD.Payment) AS TotalPayment,
   SUM( W.Invoice) / COUNT(*) AS TotalInvoice
FROM Widget W
JOIN WidgetDetail WD ON W.ID = WD.ID
GROUP BY W.[Group];