Comprehensive Income - Components of Total Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Comprehensive Income Net Of Tax [Abstract]
Net income	$ 19,660	$ 22,170	$ 22,219	$ 21,579	$ 21,214	$ 19,334	$ 21,049	$ 21,010	$ 85,628	$ 82,607	$ 75,610
Available for sale ("AFS") securities:
AFS securities with OTTI charges during the period									(7,534)	(10,840)	(33,580)
Related income tax effect									2,637	3,794	11,753
Less : OTTI charges recognized in net income									7,332	7,376	20,414
Related income tax effect									(2,566)	(2,582)	(7,145)
Reclassification of previous noncredit OTTI to credit OTTI									12,425	5,887	12,622
Related income tax effect									(4,349)	(2,061)	(4,418)
Net unrealized gains (losses) on AFS securities with OTTI									7,945	1,574	(354)
Net change in unrealized (losses) gains on AFS securities arising during the period									(1,082)	5,471	10,917
Related income tax effect									379	(1,915)	(3,821)
Net reclassification adjustment for gains included in net income									(1,216)	(16)	(1,418)
Related income tax effect									426	6	496
Total AFS securities - all other									(1,493)	3,546	6,174
Net effect of AFS securities on other comprehensive income									6,452	5,120	5,820
Held to maturity ("HTM") securities:
Unrealized loss related to the call of HTM securities transferred from AFS to the HTM portfolio									0	0	0
Related income tax effect									0	0	0
Accretion on the unrealized loss for securities transferred from AFS to the HTM investment portfolio prior to call or maturity									8	7	8
Related income tax effect									(3)	(3)	(3)
Net effect of HTM securities on other comprehensive income									5	4	5
Defined benefit pension plan:
Net actuarial loss during the period									20,305	(10,206)	(20,591)
Related income tax effect									(7,107)	3,572	7,206
Amortization of transition asset recognized in net income									0	0	0
Related income tax effect									0	0	0
Amortization of prior service cost recognized in net income									1	1	1
Related income tax effect									0	0	0
Amortization of net actuarial loss recognized in net income									4,821	4,199	2,427
Related income tax effect									(1,776)	(1,680)	(970)
Net effect of change in defined benefit pension plan on other comprehensive income									16,244	(4,114)	(11,927)
Total change in other comprehensive income, net of tax									22,701	1,010	(6,102)
Comprehensive income, net of tax									$ 108,329	$ 83,617	$ 69,508