Stockholders' Equity (Details) (USD $)	3 Months Ended									12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Components of basic and diluted earnings per share
Income from continuing operations	$ 80,893,000	$ 74,586,000	$ 82,988,000	$ 74,864,000	$ 78,759,000	$ 68,419,000	$ 51,957,000	$ 53,863,000		$ 313,331,000	$ 252,998,000	$ 192,528,000
Weighted average shares outstanding - basic										84,485,000	85,589,000	85,726,000
Dilutive potential shares from stock options												2,000
Weighted average shares outstanding - diluted										84,485,000	85,589,000	85,728,000
Earnings per share:
Earnings per share from continuing operations, basic and diluted (in dollars per share)										$ 3.71	$ 2.96	$ 2.25
Dividends
Dividends accrued, per share (in dollars per share)										$ 1.45	$ 1.18	$ 2.03
Quarterly dividend on common stock (in dollars per share)	$ 0.34									$ 0.43
Special cash dividend paid (in dollars per share)									$ 1.00
Dividends to be paid	$ 36,000,000				$ 29,000,000					$ 36,000,000	$ 29,000,000
Common stock repurchases
Shares repurchased in the open market or privately										2,252,152	1,492,535	1,536,968
Shares repurchased from employees to cover minimum income tax withholdings										599,340	665,035	568,568