Other Comprehensive Income (Loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Available-for-sale securities:
Change in net unrealized gain/loss during the period	$ (38,033)	$ 7,342	$ (43,496)	$ 6,579
Reclassification adjustment for net realized gain on sale of available-for-sale securities included in net income	(75)	(933)	(442)	(934)
Change in non-credit impairment losses on available-for-sale securities		65		65
Net unrealized losses	(38,108)	6,474	(43,938)	5,710
Employee benefit plans:
Amortization of net retirement plan actuarial gain	636	689	1,291	1,162
Amortization of net retirement plan prior service cost	14	18	29	30
Amortization of net retirement plan transition liability	13	17	25	34
Employee benefit plans	663	724	1,345	1,226
Other comprehensive (loss) income	(37,445)	7,198	(42,593)	6,936
Available-for-sale securities:
Change in net unrealized gain/loss during the period	15,209	(2,935)	17,394	(2,630)
Reclassification adjustment for net realized gain on sale of available-for-sale securities included in net income	30	372	177	372
Change in non-credit impairment losses on available-for-sale securities		(26)		(26)
Net unrealized losses	15,239	(2,589)	17,571	(2,284)
Employee benefit plans:
Amortization of net retirement plan actuarial gain	(255)	(275)	(517)	(465)
Amortization of net retirement plan prior service cost	(6)	(7)	(12)	(12)
Amortization of net retirement plan transition liability	(5)	(7)	(10)	(14)
Employee benefit plans	(266)	(289)	(539)	(491)
Other comprehensive (loss) income	14,973	(2,878)	17,032	(2,775)
Available-for-sale securities:
Change in net unrealized gain/loss during the period	(22,824)	4,407	(26,102)	3,949
Reclassification adjustment for net realized gain on sale of available-for-sale securities included in net income	(45)	(561)	(265)	(562)
Change in non-credit impairment losses on available-for-sale securities		39		39
Net unrealized losses	(22,869)	3,885	(26,367)	3,426
Employee benefit plans:
Amortization of net retirement plan actuarial gain	381	414	774	697
Amortization of net retirement plan prior service cost	8	11	17	18
Amortization of net retirement plan transition liability	8	10	15	20
Employee benefit plans	397	435	806	735
Other comprehensive (loss) income	$ (22,472)	$ 4,320	$ (25,561)	$ 4,161