Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	3 Months Ended				6 Months Ended			1 Months Ended	6 Months Ended
	Apr. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jan. 31, 2011 year	Jul. 31, 2012 Repurchase of common stock	Jun. 30, 2012 Repurchase of common stock
Shareholders' Equity
Number of shares authorized to be repurchased under the current open-market stock repurchase program								20,000,000
Repurchase of stock program, period (in years)								3
Statement
Number of shares repurchased under the current open-market stock repurchase program									488,682	3,602,307
Payments for shares repurchased under the current open-market stock repurchase program (in dollars)						$ 201,020	$ 360,998		$ 28,276	$ 201,020
Remaining number of shares authorized to be repurchased under the current open-market stock repurchase program									2,480,622
Quarter dividend paid, per share (in dollars per share)	$ 0.105
Dividends declared per common share (in dollars per share)		$ 0.105	$ 0.105	$ 0.015	$ 0.015	$ 0.210	$ 0.030
Dividend paid (in dollars)		17,048			2,608	19,500	5,241
Cumulative dividends declared		$ 16,889			$ 2,515	$ 33,938	$ 5,156