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Short-Term Debt - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Apr. 30, 2018	Dec. 31, 2017	Mar. 31, 2013
Short-term Debt [Line Items]
Trading securities pledged as collateral	$ 613,000,000		$ 613,000,000
Average balance of short-term debt	1,480,000,000		1,420,000,000
Accrued interest payable on short-term debt	3,000,000		3,000,000		$ 2,000,000
Committed line of credit	10,000,000		10,000,000
Fair value of mortgage backed securities securing loan (in excess)	4,000,000		4,000,000
Committed line of credit with financial institutions, outstanding	0		0		0
Convertible Debt \| Convertible Senior Notes Due 2018
Short-term Debt [Line Items]
Convertible notes		$ 288,000,000				$ 288,000,000
Debt Instrument interest rate		4.625%
Unamortized deferred issuance costs		$ 2,000,000
Amount of debt repurchased		37,000,000		$ 250,000,000
Gain (loss) on extinguishment of debt		$ (1,000,000)
Sequoia securities
Short-term Debt [Line Items]
Trading securities pledged as collateral	88,000,000		88,000,000
Residential loans
Short-term Debt [Line Items]
Loan pledged as collateral					1,150,000,000
Residential
Short-term Debt [Line Items]
Securities pledged as collateral	846,000,000		846,000,000		$ 788,000,000
Residential Loans
Short-term Debt [Line Items]
Loan pledged as collateral	$ 786,000,000		$ 786,000,000