Summary of Significant Accounting Policies - Agency Risk-Sharing Assets and Liabilities (Narrative) (Details) - USD ($) $ in Millions	24 Months Ended
	Dec. 31, 2015	Dec. 31, 2025	Dec. 31, 2024
Significant Accounting Policies [Line Items]
Risk-sharing arrangement term	10 years
SPE, assets		$ 19	$ 29
SPE, liabilities		$ 1	$ 3