Debt - Interest Expense on Debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Interest Costs Incurred	$ 45.3	$ 47.3	$ 47.9
Capitalization of Interest	(0.9)	(0.8)	(1.0)
Total Interest Expense	44.4	46.5	46.9
Senior Notes [Member] | Senior Notes, 6% Due November 30, 2015 [Member]
Debt Instrument [Line Items]
Interest Costs Incurred	0.0	3.7	15.5
Senior Notes [Member] | Senior Notes, 6% Due May 15, 2017 [Member]
Debt Instrument [Line Items]
Interest Costs Incurred	22.3	22.2	22.2
Senior Notes [Member] | Senior Notes, 4.35 Percent Due February 15, 2025 [Member]
Debt Instrument [Line Items]
Interest Costs Incurred	11.1	9.5	0.0
Subordinated Debt [Member] | Subordinated Debenture, 7.375 Percent Due 2054 [Member]
Debt Instrument [Line Items]
Interest Costs Incurred	11.1	11.1	9.4
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Interest Costs Incurred	0.8	0.8	0.8
Federal Home Loan Bank of Chicago [Member]
Debt Instrument [Line Items]
Interest Costs Incurred	0.0	0.0	0.0
Federal Home Loan Bank of Dallas [Member]
Debt Instrument [Line Items]
Interest Costs Incurred	$ 0.0	$ 0.0	$ 0.0