Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2007	Dec. 31, 2016	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Net operating loss carryforwards, regular tax basis	$ 1,300.0
Net operating loss carryforwards for computing the alternative minimum tax	500.0
Information regarding income tax examinations [Abstract]
Amount of IRS assessment for unpaid taxes and penalties related to REMIC issue				$ 197.5
Estimate of federal interest that may be due	191.2
Amount of payment made related to the IRS assessment on the REMIC issue		$ 65.2
Amount of IRS assessment for unpaid taxes and penalties related to disallowance of carryback of 2009 net operating loss				$ 261.4
Estimate of additional state income taxes and interest that may be due	80.9
Total amount of unrecognized tax benefits	139.9
Provision for additional taxes and interest recorded	27.2
Total amount of the unrecognized tax benefits that would affect our effective tax rate	118.4
Unrecognized tax benefits, accrued interest	$ 49.1		$ 28.9