Debt and Borrowing Arrangements - Additional Information (Details)			3 Months Ended	12 Months Ended
	Jul. 03, 2017 USD ($)	Jun. 19, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Feb. 20, 2018 USD ($)	Jan. 30, 2018 USD ($)	Jan. 01, 2018 USD ($)	Dec. 21, 2017 USD ($)	Dec. 01, 2017 USD ($)	Sep. 08, 2017 USD ($)	Jul. 31, 2017 USD ($)	Apr. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)
Debt and Borrowing Arrangements
Extinguishment of debt amount	$ 524,000,000
Loss on early extinguishment of debt			$ 34,000,000	$ 34,000,000	$ 0	$ 30,000,000
Maximum ratio of unsecured indebtedness to tangible net worth				1.25
Committed warehouse facilities
Debt and Borrowing Arrangements
Commitment period				364 days
Committed warehouse facilities | Barclays Bank PLC
Debt and Borrowing Arrangements
Borrowing capacity													$ 100,000,000
Committed warehouse facilities | Wells Fargo Bank
Debt and Borrowing Arrangements
Borrowing capacity											$ 200,000,000
Committed warehouse facilities | Fannie Mae
Debt and Borrowing Arrangements
Borrowing capacity															$ 150,000,000
Committed warehouse facilities | Bank of America
Debt and Borrowing Arrangements
Borrowing capacity										$ 75,000,000
Uncommitted warehouse facilities | Barclays Bank PLC
Debt and Borrowing Arrangements
Borrowing capacity													$ 100,000,000
Uncommitted warehouse facilities | Fannie Mae
Debt and Borrowing Arrangements
Borrowing capacity														$ 200,000,000
Variable funding notes | Wells Fargo Bank
Debt and Borrowing Arrangements
Borrowing capacity												$ 65,000,000
Warehouse Facilities
Debt and Borrowing Arrangements
Maximum ratio of unsecured indebtedness to tangible net worth				1.25
Amount to be maintained of cash and cash equivalents in excess of its liability for legal and regulatory matters				$ 100,000,000
Unsecured credit facilities
Debt and Borrowing Arrangements
Interest rate (as a percent)				0.00%
Unsecured credit facilities | Maximum
Debt and Borrowing Arrangements
Ratio of indebtedness to tangible net worth				4.50
Unsecured credit facilities | Minimum
Debt and Borrowing Arrangements
Net worth threshold, base amount				$ 250,000,000
Covenant net worth on last day of each fiscal quarter after current year				$ 150,000,000
Term notes due in 2019
Debt and Borrowing Arrangements
Extinguish of debt, cash consideration for each $1,000 principal amount		$ 1,100.00
Extinguishment of debt amount	178,000,000
Interest rate (as a percent)				7.375%
Principal amount of notes				$ 97,000,000
Term notes due in 2021
Debt and Borrowing Arrangements
Extinguish of debt, cash consideration for each $1,000 principal amount		$ 1,031.88
Extinguishment of debt amount	$ 318,000,000
Interest rate (as a percent)				6.375%
Principal amount of notes				$ 22,000,000
Subsequent Event | Committed warehouse facilities | Wells Fargo Bank
Debt and Borrowing Arrangements
Borrowing capacity							$ 100,000,000
Subsequent Event | Uncommitted warehouse facilities | Barclays Bank PLC
Debt and Borrowing Arrangements
Borrowing capacity								$ 25,000,000
Subsequent Event | Warehouse Facilities
Debt and Borrowing Arrangements
Amount to be maintained of cash and cash equivalents in excess of its liability for legal and regulatory matters									$ 70,000,000