Accumulated Other Comprehensive Loss, Net of Tax (Schedule of Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale and transferred securities:
Net unrealized loss during the period	$ (12,423)	$ (14,113)	$ (35,701)
Reclassification for net gain included in net income	0	(414)	(609)
Net non-credit other-than-temporary impairment	0	149	110
Amortization of unrealized loss on securities transferred to held-to-maturity	0	0	37
Total available-for-sale and transferred securities	(12,423)	(14,378)	(36,163)
Derivative instruments:
Net unrealized gain during the period	291	1,331	(4,945)
Reclassification adjustment for net loss included in net income	7,160	8,020	8,965
Total derivative instruments	7,451	9,351	4,020
Defined benefit pension and other postretirement benefit plans:
Current year actuarial loss	155	(368)	(8,719)
Reclassification adjustment for amortization of net loss included in net income	6,612	7,126	6,161
Reclassification adjustment for prior service cost included in net income	0	14	73
Total defined benefit pension and postretirement benefit plans	6,767	6,772	(2,485)
Available-for-sale and transferred securities:
Net unrealized loss during the period	4,833	5,212	13,166
Reclassification for net gain included in net income	0	152	223
Net non-credit other-than-temporary impairment	0	(55)	(40)
Amortization of unrealized loss on securities transferred to held-to-maturity	0	0	(14)
Total available-for-sale and transferred securities	4,833	5,309	13,335
Derivative instruments:
Net unrealized gain during the period	(110)	(506)	1,809
Reclassification adjustment for net loss included in net income	(2,776)	(2,933)	(3,279)
Total derivative instruments	(2,886)	(3,439)	(1,470)
Defined benefit pension and other postretirement benefit plans:
Current year actuarial loss	(57)	136	3,219
Reclassification adjustment for amortization of net loss included in net income	(2,575)	(2,633)	(2,274)
Reclassification adjustment for prior service cost included in net income	0	(5)	(27)
Total defined benefit pension and postretirement benefit plans	(2,632)	(2,502)	918
Available-for-sale and transferred securities:
Net unrealized loss during the period	(7,590)	(8,901)	(22,535)
Reclassification for net gain included in net income	0	(262)	(386)
Net non-credit other-than-temporary impairment	0	94	70
Amortization of unrealized loss on securities transferred to held-to-maturity	0	0	23
Total available-for-sale and transferred securities	(7,590)	(9,069)	(22,828)
Derivative instruments:
Net unrealized gains (losses) on derivative instruments	181	825	(3,136)
Reclassification adjustment for net loss included in net income	4,384	5,087	5,686
Total derivative instruments	4,565	5,912	2,550
Defined benefit pension and other postretirement benefit plans:
Current year actuarial loss	98	(232)	(5,500)
Reclassification adjustment for amortization of net loss included in net income	4,037	4,493	3,887
Reclassification adjustment for prior service cost included in net income	0	9	46
Total defined benefit pension and postretirement benefit plans	4,135	4,270	(1,567)
Other Comprehensive Income (Loss), Pre-Tax Amount	1,795	1,745	(34,628)
Other Comprehensive Income (Loss), Tax Benefit (Expense)	(685)	(632)	12,783
Other comprehensive income (loss), net of tax	$ 1,110	$ 1,113	$ (21,845)