Debt - Debt Outstanding (Details) - USD ($) $ in Millions	Dec. 31, 2019	Jul. 05, 2019	Jun. 07, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2019 - $820.2; 2018 - $911.2)	$ 778.4			$ 909.0
Secured Debt | Term Loan due June 29, 2020 (Fair Value: 2018 – $35.0)
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2019 - $820.2; 2018 - $911.2)	0.0			34.9
Secured Debt | Term Loan due July 5, 2023
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2019 - $820.2; 2018 - $911.2)	$ 49.9	$ 49.9		0.0
Senior Notes | 5.0% Senior Notes due September 19, 2022
Debt Instrument [Line Items]
Stated interest rate, percentage	5.00%
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2019 - $820.2; 2018 - $911.2)	$ 279.9			281.5
Senior Notes | Senior Notes Payable, 4.35% due 2025 (Fair Value: 2019 – $478.6; 2018 – $444.2)
Debt Instrument [Line Items]
Stated interest rate, percentage	4.35%
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2019 - $820.2; 2018 - $911.2)	$ 448.6			448.4
Subordinated Debt | Subordinated Debentures due 2054 (Fair Value: 2018 – $151.1)
Debt Instrument [Line Items]
Stated interest rate, percentage	7.375%		7.375%
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2019 - $820.2; 2018 - $911.2)	$ 0.0			$ 144.2