Stockholders' Equity (Details) (USD $)	3 Months Ended												12 Months Ended			192 Months Ended
	Dec. 31, 2012 item	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012
Stockholders' Equity
Dividends paid (in dollars per share)	$ 0.085	$ 0.085	$ 0.085	$ 0.085	$ 0.080	$ 0.080	$ 0.080	$ 0.080	$ 0.075	$ 0.075	$ 0.075	$ 0.075	$ 0.43	$ 0.32	$ 0.30
Frequency of dividends paid	2
Common stock received by Company in lieu of cash from stock option exercises and vesting of non-vested shares (in shares)													22,168			1,900,000
Common stock received by Company in lieu of cash from stock option exercises and vesting of non-vested shares													$ 200,000
Stock Repurchase program
Common stock repurchased													4,402,000	8,363,000
2013 dividends
Planned Dividends
Planned quarterly dividend for second, third, and fourth quarters (in dollars per share)	$ 0.085												$ 0.085			0.085
Stock Repurchase programs
Stock Repurchase program
Common stock repurchased (in shares)																65,600,000
Common stock repurchased																1,200,000,000
2012 Stock Repurchase program
Stock Repurchase program
Common stock repurchased (in shares)													700,000
Common stock repurchased													4,400,000
Authorized amount to repurchase shares													10,000,000
Amount available to repurchase additional shares													$ 5,600,000