Income Taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Tax provision at U.S. federal statutory rate	$ 253	$ 157	$ 517
Tax-exempt interest	(123)	(124)	(132)
Decrease in deferred tax valuation allowance	0	(79)	(102)
Stock-based compensation	(15)	0	0
Solar credits	0	(79)	0
Sale of HFPI and foreign rate differential	5	(37)	0
Tax Reform	877	0	0
Other	(12)	(4)	6
Total income tax expense (benefit)	$ 985	$ (166)	$ 289