Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of Significant Accounting Policies [Line Items]
Provision for U.S federal income taxes	$ (2,346,000,000)	$ 82,000,000	$ 118,000,000
U.S. Federal Tax [Member] | REIT [Member]
Summary of Significant Accounting Policies [Line Items]
Provision for U.S federal income taxes	$ 0
Computer Equipment and Acquired Software [Member]
Summary of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	3 years
Minimum [Member] | Buildings and Improvements [Member]
Summary of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	8 years
Minimum [Member] | Furniture and Equipment [Member]
Summary of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	3 years
Maximum [Member] | Buildings and Improvements [Member]
Summary of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	40 years
Maximum [Member] | Furniture and Equipment [Member]
Summary of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	8 years