Fair Value of Financial Instruments (Assets and Liabilities Measured at Fair Value on a Non-Recurring Basis) (Narrative) (Details) - 4.875% Senior Notes [Member] - Senior Notes [Member] - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
Debt instrument, interest rate (percentage)	4.875%
Fair Value Measurements, Non-Recurring [Member] | Gross Carrying Amount [Member]
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
4.875% Senior Notes (in dollars)	$ 400.0	$ 400.0
Fair Value Measurements, Non-Recurring [Member] | Fair Value [Member] | Level 1 [Member]
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
4.875% Senior Notes (in dollars)	$ 400.0	$ 410.0