Debt - Debt Outstanding (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2016 - $770.9; 2015 - $781.3)	$ 751.6	$ 750.6
Senior Notes [Member] | Senior Notes, 6% Due May 15, 2017 [Member]
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2016 - $770.9; 2015 - $781.3)	359.8	359.1
Senior Notes [Member] | Senior Notes, 4.35 Percent Due February 15, 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2016 - $770.9; 2015 - $781.3)	247.7	247.4
Subordinated Debt [Member] | Subordinated Debenture, 7.375 Percent Due 2054 [Member]
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2016 - $770.9; 2015 - $781.3)	$ 144.1	$ 144.1