Stock-Based Compensation (Narrative) (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense on restricted stock awards	$ 15.9
Unrecognized compensation expense on restricted stock awards	$ 43.9
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Requisite Service Period	2.5
Shares awarded	961	[1]
Time Vesting Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded	500
Performance Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded	500

[1] Weighted-average grant-date fair value of $38.34 per share and a weighted-average requisite service period of 2.5 years. The awards with market conditions included an expected volatility of 31% in the Monte Carlo simulation used to measure grant-date fair value.