Debt - Summary of Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,189.5	$ 1,223.0
Deferred issuance costs	10.8	11.2
Total long-term debt	1,178.7	1,211.8
Interest payments made	49.5	65.7
Federal Home Loan Bank Advances (FHLB)
Debt Instrument [Line Items]
Long-term debt, gross	155.0	0.0
Interest payments made	2.4	0.0
5% Convertible Senior Notes due 2017
Debt Instrument [Line Items]
Long-term debt, gross	145.0	333.5
Total long-term debt		331.5
Interest payments made	10.6	17.3
2% Convertible Senior Notes due 2020
Debt Instrument [Line Items]
Long-term debt, gross	207.6	500.0
Total long-term debt		490.8
Interest payments made	9.1	10.0
Senior Notes | 5.75% Senior Notes due 2023
Debt Instrument [Line Items]
Long-term debt, gross	425.0	0.0
Interest payments made	0.0	0.0
Convertible Junior Subordinated Debentures Due 2063
Debt Instrument [Line Items]
Long-term debt, gross	256.9	389.5
Total long-term debt		389.5
Interest payments made	27.4	35.1
Senior Notes
Debt Instrument [Line Items]
Interest payments made	$ 0.0	$ 3.3