Stockholders' Equity (Details) (USD $)	3 Months Ended					12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Components of basic and diluted earnings per share
Income from continuing operations		$ 52,350,000	$ 52,116,000	$ 41,225,000	$ 46,837,000	$ 252,998,000	$ 192,528,000	$ 172,205,000
Weighted average shares outstanding - basic						85,589,000	85,726,000	85,783,000
Dilutive potential shares from stock options							2,000	10,000
Weighted average shares outstanding - diluted						85,589,000	85,728,000	85,793,000
Earnings per share:
Earnings per share from continuing operations, basic and diluted (in dollars per share)		$ 0.61	$ 0.61	$ 0.48	$ 0.55	$ 2.96	$ 2.25	$ 2.01
Anti-dilutive Securities
Anti-dilutive securities excluded from the diluted earnings per share (in shares)						0	0	16,000
Dividends
Dividends accrued, per share (in dollars per share)						$ 1.18	$ 2.03	$ 0.85
Special cash dividend paid (in dollars per share)		$ 1.00
Quarterly dividend on common stock (in dollars per share)	$ 0.28					$ 0.34
Dividends to be paid	$ 29,000,000	$ 24,000,000				$ 29,000,000	$ 24,000,000
Common stock repurchases
Shares repurchased in the open market or privately						1,492,535	1,536,968	1,951,331
Shares repurchased from employees to cover minimum income tax withholdings						665,035	568,568	494,207