Debt - Long Term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2019 - $798.5; 2018 - $911.2)	$ 873.3	$ 909.0
Senior Notes | Term Loan due June 29, 2020
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2019 - $798.5; 2018 - $911.2)	0.0	34.9
Senior Notes | 5.0% Senior Notes due September 19, 2022
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2019 - $798.5; 2018 - $911.2)	280.6	281.5
Senior Notes | 4.35% Senior Notes due February 15, 2025
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2019 - $798.5; 2018 - $911.2)	448.5	448.4
Subordinated Debt | 7.375% Subordinated Debentures due February 27, 2054
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2019 - $798.5; 2018 - $911.2)	$ 144.2	$ 144.2