Debt (Issuance of December 2015 Senior Notes) (Details) $ in Millions			12 Months Ended
		Dec. 09, 2015 USD ($)	Dec. 31, 2015 USD ($)		Dec. 31, 2016 USD ($)		Nov. 09, 2010
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage							6.00%
Senior Notes | 2018 Senior Notes (3.000% coupon)
Debt Instrument [Line Items]
Long-term debt		$ 500.0	$ 499.4	[1]	$ 499.6	[1]
Debt instrument, interest rate, stated percentage	[1]	3.00%
Debt Instrument, Basis Spread on Variable Rate, Basis Points		30
Debt Instrument, Unamortized Discount			0.6
Senior Notes | 2020 Senior Notes (3.750% coupon)
Debt Instrument [Line Items]
Long-term debt		$ 500.0	499.8	[2]	$ 499.9	[2]
Debt instrument, interest rate, stated percentage	[2]	3.75%
Debt Instrument, Basis Spread on Variable Rate, Basis Points		35
Debt Instrument, Unamortized Discount			0.2
Senior Notes | December 2015 Senior Notes
Debt Instrument [Line Items]
Debt Related Commitment Fees and Debt Issuance Costs			$ 4.7

[1] Instrument is callable by the Company at any time at the greater of 100% of the principal amount and the sum of the present values of the remaining scheduled payments of principal and interest discounted at the U.S. Treasury rate plus 0.30% plus, in each case, accrued and unpaid interest.

[2] Instrument is callable by the Company at any time prior to the date that is one month prior to the instrument’s maturity date at the greater of 100% of the principal amount and the sum of the present values of the remaining scheduled payments of principal and interest discounted at the U.S. Treasury rate plus 0.35% plus, in each case, accrued and unpaid interest. On or after such date, the instrument is callable by the Company at 100% of the principal amount plus accrued and unpaid interest.