Debt - Summary of Debt Obligations (Details) $ / shares in Units, $ in Millions	1 Months Ended	9 Months Ended
	Apr. 30, 2017	Sep. 30, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($)	Aug. 31, 2016
Debt Instrument [Line Items]
Long-term debt, par value		$ 836.9	$ 1,189.5
Debt issuance costs		(6.8)	(10.8)
Long-term debt, carrying value		830.1	1,178.7
FHLB Advance
Debt Instrument [Line Items]
Long-term debt, par value		155.0	155.0
5% Convertible Senior Notes due 2017
Debt Instrument [Line Items]
Long-term debt, par value		$ 0.0	145.0
Stated interest rate (in hundredths)		5.00%
2% Notes
Debt Instrument [Line Items]
Long-term debt, par value		$ 0.0	207.6
Stated interest rate (in hundredths)		2.00%
Conversion rate (in shares per $1,000 note)	0.1438332
5.75% Notes | 5.75% Notes
Debt Instrument [Line Items]
Long-term debt, par value		$ 425.0	425.0
Stated interest rate (in hundredths)		5.75%		5.75%
9% Debentures
Debt Instrument [Line Items]
Long-term debt, par value		$ 256.9	$ 256.9
Stated interest rate (in hundredths)		9.00%
Conversion rate (in shares per $1,000 note)		0.0740741
Convertible debt, conversion price (in dollars per share) | $ / shares		$ 13.50
Period preceding election to convert (in days)		5 days