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Debt Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 05, 2014
Debt Instrument [Line Items]
Repayments of Debt				$ 589.9
Cash payments paid for interest			$ 58.6	50.0
Amortization of net (loss) gain on treasury lock	$ 1.4	$ 1.4	4.2	4.2
Amortization of financing costs	0.5	2.0	1.3	$ 2.9
Deferred finance costs expensed		$ 1.5
Book value of fixed rate debt	1,647.2		1,647.2
Book value of variable rate debt	678.6		678.6
Long-term debt (fixed-rate debt), fair value	$ 1,711.9		1,711.9
Unsecured Debt \| Term loan, Due October 2018 [Member]
Debt Instrument [Line Items]
Repayment of debt			$ 25.0
Debt Instrument, Term			5 years
Unsecured Debt \| Seven-Year Term Loan, due October 2020
Debt Instrument [Line Items]
Repayment of debt			$ 4.9
Debt Instrument, Term			7 years
Senior Notes [Member] \| 3.65% Senior Notes, due September 2024 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount					$ 400.0
Debt Instrument, Interest Rate, Stated Percentage					3.65%