Reconciliation of Income (Loss) from Continuing Operations, Income (Loss) from Discontinued Operations and Net Income (Loss) and Weighted Average Common Shares Outstanding for Calculating Diluted Earnings (Loss) Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Earnings Per Share Diluted [Line Items]
Total income from continuing operations available to common shareholders, basic	$ 156,565		$ 149,849		$ 335,797		$ 309,601
Income (loss) from discontinued operations, net of tax	(4,423)		756		(6,288)		(152)
Total income available to common shareholders, basic	152,142		150,605		329,509		309,449
Undistributed earnings for participating securities	26		41		49		81
Total income from continuing operations available to common shareholders, diluted	156,591		149,890		335,846		309,682
Total income available to common shareholders, diluted	$ 152,168		$ 150,646		$ 329,558		$ 309,530
Weighted-average common shares outstanding, basic	178,992		177,965		178,800		177,671
Weighted-average common shares outstanding, diluted	179,948		179,154		179,723		178,906
Diluted earnings per share:
Income from continuing operations	$ 0.87	[1],[2]	$ 0.84	[1],[2]	$ 1.87	[1],[2]	$ 1.73	[1],[2]
Income (loss) from discontinued operations, net of tax	$ (0.02)	[1],[2]	$ 0.00	[1],[2]	$ (0.03)	[1],[2]	$ 0.00	[1],[2]
Net income	$ 0.85	[1],[2]	$ 0.84	[1],[2]	$ 1.83	[1],[2]	$ 1.73	[1],[2]
Restricted Stock Units
Earnings Per Share Diluted [Line Items]
Employee stock purchase plan	497		502		460		473
Stock Options
Earnings Per Share Diluted [Line Items]
Employee stock purchase plan	458		685		462		760
Employee Stock Purchase Plan
Earnings Per Share Diluted [Line Items]
Employee stock purchase plan	1		2		1		2

[1] Amounts may not sum due to rounding

[2] Earnings per share amounts are computed independently for income from continuing operations, income (loss) from discontinued operations and net income. As a result, the sum of per-share amounts from continuing operations and discontinued operations may not equal the total per-share amount for net income.