Income Taxes (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2007	Dec. 31, 2016	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Net operating loss carryforwards, regular tax basis	$ 987,600,000
Net operating loss carryforwards for computing the alternative minimum tax	117,900,000
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	200,300,000
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	84,100,000
Total amount of unrecognized tax benefits	141,800,000
Provision for additional taxes and interest recorded	28,400,000
Total amount of the unrecognized tax benefits that would affect our effective tax rate	119,800,000
Unrecognized tax benefits, accrued interest	$ 50,800,000		$ 28,900,000