Shareholders' Equity - Net Change in Components of Accumulated Other Comprehensive Income (Loss) and the Related Tax Effects (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Securities available for sale and transferred securities, Before tax amount [Abstract]
Net unrealized holding gains (losses) arising during the period, Before tax amount	$ 4,585	$ 9,372	$ 6,870	$ 44,714
Reclassification adjustment for net (gains) losses realized in net income, Before tax amount	(1)	0	(1)	310
Change in net unrealized holding loss on securities transferred to held to maturity, Before tax amount	1,189	2,973	2,421	5,697
Total securities available for sale and transferred securities, Before tax amount	5,773	12,345	9,290	50,721
Securities available for sale and transferred securities, Tax (expense) benefit [Abstract]
Net unrealized holding gains (losses) arising during the period, Tax (expense) benefit	(1,753)	(3,585)	(2,627)	(17,102)
Reclassification adjustment for net (gains) losses realized in net income, Tax (expense) benefit	0	0	0	(119)
Change in net unrealized holding loss on securities transferred to held to maturity, Tax (expense) benefit	(455)	(1,137)	(926)	(2,179)
Total securities available for sale and transferred securities, Tax (expense) benefit	(2,208)	(4,722)	(3,553)	(19,400)
Securities available for sale and transferred securities, Net of tax amount [Abstract]
Net unrealized holding gains (losses) arising during the period, net of tax amount	2,832	5,787	4,243	27,612
Reclassification adjustment for net (gains) losses realized in net income, net of tax amount	(1)	0	(1)	191
Change in net unrealized holding loss on securities transferred to held to maturity, net of tax amount	734	1,836	1,495	3,518
Total securities available for sale and transferred securities, net of tax amount	3,565	7,623	5,737	31,321
Pension and other postretirement benefit plans, Before tax amount [Abstract]
Net change in prior service costs, before tax amount	62	62	125	125
Recognized net loss due to lump sum settlements, before tax amount	0	2,248	0	2,671
Change in net actuarial loss, before tax amount	780	874	1,567	1,756
Recognized net loss due to defined benefit plan termination, before tax amount	16,991	0	16,991	0
Total pension and other postretirement benefit plans, before tax amount	17,833	3,184	18,683	4,552
Pension and other postretirement benefit plans, Tax (expense) benefit [Abstract]
Net change in prior service costs, tax (expense) benefit	(24)	(23)	(48)	(48)
Recognized net loss due to lump sum settlements, tax (expense) benefit	0	(860)	0	(1,022)
Change in net actuarial loss, tax (expense) benefit	(298)	(334)	(599)	(671)
Recognized net loss due to defined benefit plan termination, tax (expense) benefit	(6,499)	0	(6,499)	0
Total pension and other postretirement benefit plans, tax (expense) benefit	(6,821)	(1,217)	(7,146)	(1,741)
Pension and other postretirement benefit plans, Net of tax amount [Abstract]
Net change in prior service costs, net of tax amount	38	39	77	77
Recognized net loss due to lump sum settlements, net of tax amount	0	1,388	0	1,649
Change in net actuarial loss, net of tax amount	482	540	968	1,085
Recognized net loss due to defined benefit plan termination, net of tax amount	10,492	0	10,492	0
Total pension and other postretirement benefit plans, net of tax amount	11,012	1,967	11,537	2,811
Cash flow hedge derivatives, Before tax amount [Abstract]
Change in accumulated gain (loss) on effective cash flow hedge derivatives, before tax amount	(209)	(449)	(152)	(1,777)
Reclassification adjustment for loss realized in net income, before tax amount	78	156	177	316
Total cash flow hedge derivatives, before tax benefit	(131)	(293)	25	(1,461)
Total other comprehensive income (loss), Before tax amount	23,475	15,236	27,998	53,812
Cash flow hedge derivatives, Tax (expense) benefit [Abstract]
Change in accumulated gain (loss) on effective cash flow hedge derivatives , tax (expense) benefit	80	172	58	680
Reclassification adjustment for loss realized in net income, tax (expense) benefit	(30)	(60)	(68)	(121)
Total cash flow hedge derivatives, tax (expense) benefit	50	112	(10)	559
Total other comprehensive income (loss), Tax expense (benefit)	(8,979)	(5,827)	(10,709)	(20,582)
Cash flow hedge derivatives, Net of tax amount [Abstract]
Change in accumulated gain (loss) on effective cash flow hedge derivatives, net of tax amount	(129)	(277)	(94)	(1,097)
Reclassification adjustment for loss realized in net income, net of tax amount	48	96	109	195
Total cash flow hedge derivatives, net of tax amount	(81)	(181)	15	(902)
Total other comprehensive income (loss)	$ 14,496	$ 9,409	$ 17,289	$ 33,230