Accumulated Other Comprehensive Income Amounts Reclassified (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income (loss) before income taxes	$ 101,169			$ 95,985			$ 142,558	$ 130,027	$ 123,214	$ 137,507
Income tax expense (benefit)	(23,289)			(22,714)			(32,463)	(30,427)	(28,404)	(31,407)
Net income	77,880	$ 26,126	$ 6,089	73,271	$ 33,295	$ (6,966)	110,095	99,600	94,810	106,100
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	2,185			2,413			6,555	7,243	8,969	9,657
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	(854)			(1,640)			(2,562)	(4,790)	(4,159)	(6,483)
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	1,331			773			3,993	2,453	4,810	3,174
Reclassification from AOCI, Current Period, Tax	(301)			(174)			(862)	(520)	(1,068)	(872)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1,030			599			3,131	1,933	3,742	2,302
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	2,487			(364)			1,714	(883)	1,461	338
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(120)			(112)			(337)	(310)	(450)	(397)
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	3,397			123			4,508	740	4,753	2,243
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net realized gain (loss) on sale of securities	3,202			(443)			2,232	(1,108)	1,895	396
Income (loss) before income taxes	3,202			(443)			2,232	(1,108)	1,895	396
Income tax expense (benefit)	(715)			79			(518)	225	(434)	(58)
Net income	2,487			(364)			1,714	(883)	1,461	338
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income (loss) before income taxes	(153)			(144)			(451)	(423)	(596)	(559)
Income tax expense (benefit)	33			32			114	113	146	162
Net income	(120)			(112)			(337)	(310)	(450)	(397)
Interest Rate Contract | Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest on long-term debt and revolving credit facility	$ (153)			$ (144)			$ (451)	$ (423)	$ (596)	$ (559)