Shareholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended		3 Months Ended				9 Months Ended		1 Months Ended	9 Months Ended
	Apr. 30, 2012	Jan. 31, 2011	Sep. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Oct. 31, 2012 Repurchase of common stock	Sep. 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		3 years
Statement
Number of shares repurchased under the current open-market stock repurchase program									1,396,244	4,093,416
Payments for shares repurchased under the current open-market stock repurchase program (in dollars)							$ 229,442	$ 534,000	$ 84,611	$ 229,443
Remaining number of shares authorized to be repurchased under the current open-market stock repurchase program									1,081,951
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	$ 0.315	$ 0.045
Dividend paid (in dollars)			16,885			2,547	36,385	7,788
Cumulative dividends declared			$ 16,964			$ 2,494	$ 50,905	$ 7,650