Earnings Per Share (Reconciliation Of Basic And Diluted Earnings Per Share) (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share [Line Items]
Net income	$ 30,952		$ 29,657		$ 29,803		$ 28,797		$ 28,124		$ 23,263		$ 22,208		$ 21,672		$ 119,209	$ 95,267	$ 102,226
Weighted average basic common shares outstanding																	40,332	40,071	39,828
Effect of dilutive securities-stock options and awards																	191	251	232
Total shares and dilutive securities																	40,523	40,322	40,060
Basic earnings per common share	$ 0.77	[1]	$ 0.73	[1]	$ 0.74	[1]	$ 0.72	[1]	$ 0.70	[1]	$ 0.58	[1]	$ 0.55	[1]	$ 0.54	[1]	$ 2.96	$ 2.38	$ 2.57
Diluted earnings per common share	$ 0.76	[1]	$ 0.73	[1]	$ 0.74	[1]	$ 0.71	[1]	$ 0.70	[1]	$ 0.58	[1]	$ 0.55	[1]	$ 0.54	[1]	$ 2.94	$ 2.36	$ 2.55
Antidilutive stock options and awards																	409	277	284

[1] The sum of the quarterly per share amounts may not equal the annual amount reported, as per share amounts are computed independently for each quarter and for the full year.