Stockholders' Equity (Details) (USD $)	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Components of basic and diluted earnings per share
Net income (in dollars)	$ 40,022,000	$ 39,834,000	$ 49,970,000	$ 45,633,000	$ 46,365,000	$ 40,533,000	$ 34,152,000	$ 35,909,000		$ 175,459,000	$ 156,959,000	$ 105,505,000
Weighted average shares outstanding - basic										85,783,000	85,618,000	85,484,000
Dilutive potential shares from stock options										10,000	29,000	60,000
Weighted average shares outstanding - diluted										85,793,000	85,647,000	85,544,000
Earnings per share:
Basic (in dollars per share)	$ 0.47	$ 0.46	$ 0.58	$ 0.53	$ 0.54	$ 0.47	$ 0.40	$ 0.42		$ 2.05	$ 1.83	$ 1.23
Diluted (in dollars per share)	$ 0.47	$ 0.46	$ 0.58	$ 0.53	$ 0.54	$ 0.47	$ 0.40	$ 0.42		$ 2.05	$ 1.83	$ 1.23
Anti-dilutive Securities
Anti-dilutive securities excluded from the diluted earnings per share (in shares)										16,000	203,000	777,000
Dividends
Dividend approved on common stock (in dollars per share)										$ 0.85	$ 0.77	$ 0.76
Dividends to be paid, included in other current liabilities (in dollars)	$ 21,400,000				$ 17,100,000					$ 21,400,000	$ 17,100,000
Quarterly dividend on common stock (in dollars per share)	$ 0.25								$ 0.20
Common stock repurchases
Shares repurchased in the open market or privately										1,951,331	2,043,545	1,870,034
Shares repurchased from employees to cover their minimum income tax withholdings										494,207	426,665	327,301