Stockholders' Equity Narrative (Details) (USD $) Share data in Millions, unless otherwise specified	1 Months Ended		3 Months Ended		6 Months Ended
	Jan. 31, 2013	Apr. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Stockholders' Equity Note [Abstract]
Stock repurchase program, authorized amount	$ 150,000,000	$ 150,000,000
Shares repurchased during period (in shares)			1.1	2.2	2.2	2.4
Value of shares repurchased during period			42,500,000	67,200,000	82,800,000	75,100,000
Remaining amount available for future purchases of shares under approved repurchase program					76,800,000
Amortization expense from capitalized stock-based compensation			$ 2,000,000	$ 1,900,000	$ 3,900,000	$ 3,700,000