Accumulated Other Comprehensive Income Amounts Reclassified (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income (loss) before income taxes	$ 95,985	$ 92,216	$ 130,027	$ 141,785	$ 137,507	$ 149,936
Income tax expense (benefit)	(22,714)	(32,532)	(30,427)	(50,024)	(31,407)	(52,519)
Net income	73,271	59,684	99,600	91,761	106,100	97,417
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	2,413	2,414	7,243	7,243	9,657	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	(1,640)	(1,695)	(4,790)	(5,083)	(6,483)	(6,516)
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	773	719	2,453	2,160	3,174	3,141
Reclassification from AOCI, Current Period, Tax	(174)	(262)	(520)	(716)	(872)	(1,742)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	599	457	1,933	1,444	2,302	1,399
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(364)	1,413	(883)	7,303	338	9,016
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	(112)	(85)	(310)	(222)	(397)	(303)
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	123	1,785	740	8,525	2,243	10,112
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	(443)	1,765	(1,108)	9,122	396	11,192
Income (loss) before income taxes	(443)	1,765	(1,108)	9,122	396	11,192
Income tax expense (benefit)	79	(352)	225	(1,819)	(58)	(2,176)
Net income	(364)	1,413	(883)	7,303	338	9,016
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	(144)	(134)	(423)	(396)	(559)	(523)
Income tax expense (benefit)	32	49	113	174	162	220
Net income	(112)	(85)	(310)	(222)	(397)	(303)
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	$ (144)	$ (134)	$ (423)	$ (396)	$ (559)	$ (523)