Subsequent Events (Details) - USD ($)			12 Months Ended
	Jan. 31, 2018	Jan. 22, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Decrease in other assets related to BlackRock Series C Preferred Stock transferred			$ (757,000,000)	$ 644,000,000	$ (1,568,000,000)
BlackRock Series C Preferred Stock
Debt Instrument [Line Items]
LTIP - Number of preferred stock shares transferred (shares)			520,000
LTIP - Total preferred shares held at end of period (shares)			250,000
Subsequent Event
Debt Instrument [Line Items]
Decrease in other assets related to BlackRock Series C Preferred Stock transferred	$ (42,000,000)
Decrease in other liabilities related to BlackRock Series C Preferred Stock transferred	$ (42,000,000)
Subsequent Event | BlackRock Series C Preferred Stock
Debt Instrument [Line Items]
LTIP - Number of preferred stock shares transferred (shares)	103,064
LTIP - Total preferred shares held at end of period (shares)	143,458
Subsequent Event | Senior Notes | Senior Notes Maturing January 2021
Debt Instrument [Line Items]
Debt issued		$ 900,000,000
Debt Instrument, Maturity Date		Jan. 22, 2021
Stated interest rate		2.50%
Subsequent Event | Senior Notes | Senior Notes Maturing January 2028
Debt Instrument [Line Items]
Debt issued		$ 700,000,000
Debt Instrument, Maturity Date		Jan. 22, 2028
Stated interest rate		3.25%
Subsequent Event | Senior Notes | Senior Floating Rate Notes Maturing January 2021
Debt Instrument [Line Items]
Debt issued		$ 400,000,000
Debt Instrument, Maturity Date		Jan. 22, 2021
Subsequent Event | Senior Notes | Senior Floating Rate Notes Maturing January 2021 | 3-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate - LIBOR Plus		0.25%