Summary of Significant Accounting Policies - NPC - Reconciliation of Cash (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 1,565	$ 1,591
Investments and restricted cash and cash equivalents	224	173
Total cash and cash equivalents and restricted cash and cash equivalents	1,811	1,817	$ 1,244	$ 1,445
NPC
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	20	43
Investments and restricted cash and cash equivalents	17	17
Total cash and cash equivalents and restricted cash and cash equivalents	$ 37	$ 60	$ 45	$ 36