Income Taxes (Details) - USD ($)	12 Months Ended
	Dec. 31, 2007	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Net operating loss carryforwards		$ 585,700,000
Valuation allowance		0	$ 0
Information regarding income tax examinations [Abstract]
Amount of IRS assessment for unpaid taxes and penalties related to REMIC issue				$ 197,500,000
Estimate of federal interest that may be due		209,700,000
Amount of payment made related to the IRS assessment on the REMIC issue	$ 65,200,000
Amount of IRS assessment for unpaid taxes and penalties related to disallowance of carryback of 2009 net operating loss				$ 261,400,000
Estimate of additional state income taxes and interest that may be due		87,400,000
Total amount of unrecognized tax benefits		143,700,000
Total amount of the unrecognized tax benefits that would affect our effective tax rate		125,000,000
Unrecognized tax benefits, accrued interest		$ 52,900,000	$ 52,000,000