Accumulated Other Comprehensive Income (Loss) - Accumulated Other Comprehensive Income, Changes In (Details) - USD ($)				3 Months Ended		9 Months Ended
		Jan. 01, 2019	Jan. 01, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		$ 731,291,000	$ 754,140,000	$ 787,765,000	$ 751,810,000	$ 731,291,000	$ 754,140,000
Cumulative effect of accounting change		(16,452,000)	0
Adjusted beginning balance, net of tax		714,839,000	754,140,000
Income tax (expense) benefit				(3,125,000)	3,102,000	(17,789,000)	8,713,000
Ending balance, net of tax				810,453,000	752,506,000	810,453,000	752,506,000
Retained Earnings
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		64,797,000	32,851,000	65,183,000	48,843,000	64,797,000	32,851,000
Cumulative effect of accounting change		(16,452,000)	(85,000)
Adjusted beginning balance, net of tax		48,345,000	32,766,000
Ending balance, net of tax				74,477,000	58,578,000	74,477,000	58,578,000
Unrealized Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		(31,120,000)	(16,295,000)	31,070,000	(42,536,000)	(31,120,000)	(16,295,000)
Adjusted beginning balance, net of tax			(16,210,000)
Other comprehensive income (loss) before reclassifications				17,334,000	(16,885,000)	96,156,000	(52,757,000)
Reclassification adjustments included in net income				70,000	80,000	(31,000)	2,629,000
Income tax (expense) benefit				(3,655,000)	3,529,000	(20,186,000)	10,526,000
Net current-period other comprehensive income (loss), net of tax				13,749,000	(13,276,000)	75,939,000	(39,602,000)
Ending balance, net of tax				44,819,000	(55,812,000)	44,819,000	(55,812,000)
Unrealized Gains (Losses) on Derivatives
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		7,146,000	6,399,000	(820,000)	10,753,000	7,146,000	6,399,000
Adjusted beginning balance, net of tax			6,399,000
Other comprehensive income (loss) before reclassifications				(2,619,000)	1,894,000	(11,392,000)	7,864,000
Reclassification adjustments included in net income				(499,000)	(406,000)	(1,809,000)	(864,000)
Income tax (expense) benefit				655,000	(312,000)	2,772,000	(1,470,000)
Net current-period other comprehensive income (loss), net of tax				(2,463,000)	1,176,000	(10,429,000)	5,530,000
Ending balance, net of tax				(3,283,000)	11,929,000	(3,283,000)	11,929,000
Net Prior Service (Cost) Credit
AOCI Attributable to Parent, Net of Tax
Reclassification adjustments included in net income	[1]			(1,000)	(1,000)	(5,000)	(5,000)
Net Gain (Loss)
AOCI Attributable to Parent, Net of Tax
Reclassification adjustments included in net income	[1]			596,000	547,000	1,789,000	1,642,000
Total
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		(50,228,000)	(36,298,000)	4,935,000	(57,322,000)	(50,228,000)	(36,298,000)
Cumulative effect of accounting change			85,000
Adjusted beginning balance, net of tax		(50,228,000)	(36,213,000)
Other comprehensive income (loss) before reclassifications				14,715,000	(14,991,000)	84,764,000	(44,893,000)
Reclassification adjustments included in net income				166,000	220,000	(56,000)	3,402,000
Income tax (expense) benefit				(3,125,000)	3,102,000	(17,789,000)	8,713,000
Net current-period other comprehensive income (loss), net of tax				11,756,000	(11,669,000)	66,919,000	(32,778,000)
Ending balance, net of tax				16,691,000	(68,991,000)	16,691,000	(68,991,000)
Accounting Standards Update 2016-01 | Retained Earnings
AOCI Attributable to Parent, Net of Tax
Cumulative effect of accounting change			(85,000)
Cumulative Effect on Retained Earnings, before Tax			(107,000)
Accounting Standards Update 2016-01 | Unrealized Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax
Cumulative effect of accounting change	[2]		85,000
Accounting Standards Update 2016-01 | Unrealized Gains (Losses) on Derivatives
AOCI Attributable to Parent, Net of Tax
Cumulative effect of accounting change			0
Accounting Standards Update 2016-01 | Total
AOCI Attributable to Parent, Net of Tax
Cumulative effect of accounting change	[2]		85,000
Defined Benefit Pension Plan | Net Prior Service (Cost) Credit
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		(139,000)	(133,000)	(142,000)	(135,000)	(139,000)	(133,000)
Adjusted beginning balance, net of tax			(133,000)
Other comprehensive income (loss) before reclassifications				0	0	0	0
Reclassification adjustments included in net income				(1,000)	(1,000)	(5,000)	(5,000)
Income tax (expense) benefit				0	0	1,000	2,000
Net current-period other comprehensive income (loss), net of tax				(1,000)	(1,000)	(4,000)	(3,000)
Ending balance, net of tax				(143,000)	(136,000)	(143,000)	(136,000)
Defined Benefit Pension Plan | Net Gain (Loss)
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		$ (26,115,000)	(26,269,000)	(25,173,000)	(25,404,000)	(26,115,000)	(26,269,000)
Adjusted beginning balance, net of tax			(26,269,000)
Other comprehensive income (loss) before reclassifications				0	0	0	0
Reclassification adjustments included in net income				596,000	547,000	1,789,000	1,642,000
Income tax (expense) benefit				(125,000)	(115,000)	(376,000)	(345,000)
Net current-period other comprehensive income (loss), net of tax				471,000	432,000	1,413,000	1,297,000
Ending balance, net of tax				$ (24,702,000)	$ (24,972,000)	$ (24,702,000)	$ (24,972,000)
Defined Benefit Pension Plan | Accounting Standards Update 2016-01 | Net Prior Service (Cost) Credit
AOCI Attributable to Parent, Net of Tax
Cumulative effect of accounting change			0
Defined Benefit Pension Plan | Accounting Standards Update 2016-01 | Net Gain (Loss)
AOCI Attributable to Parent, Net of Tax
Cumulative effect of accounting change			$ 0

[1] These accumulated other comprehensive income (“AOCI”) components are included in the computation of net periodic pension cost (income) presented in “Note 9 - Employee Benefit Plans.”

[2] The Company adopted ASU 2016-01 on January 1, 2018. This amount includes a reclassification for the cumulative adjustment to retained earnings of $107,000 ($85,000, net of tax).