Accumulated Other Comprehensive Income Amounts Reclassified (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income (loss) before income taxes		$ 92,216	$ 117,770	$ 141,785	$ 142,535	$ 149,936	$ 142,989
Income tax expense (benefit)		(32,532)	(43,134)	(50,024)	(51,423)	(52,519)	(51,229)
Net income (loss)		59,684	74,636	91,761	91,112	97,417	91,760
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	[1]	2,414	1,663	7,243	4,993	9,657	6,636
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]	(1,695)	(1,087)	(5,083)	(3,532)	(6,516)	(5,688)
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1]	719	576	2,160	1,461	3,141	948
Reclassification from AOCI, Current Period, Tax		(262)	228	(716)	6	(1,742)	60
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		457	804	1,444	1,467	1,399	1,008
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		1,413	1,687	7,303	4,493	9,016	7,096
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		(85)	39	(222)	(78)	(303)	(155)
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		1,785	2,530	8,525	5,882	10,112	7,949
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net realized gain on sale of securities		1,765	2,072	9,122	5,570	11,192	8,797
Income (loss) before income taxes		1,765	2,072	9,122	5,570	11,192	8,797
Income tax expense (benefit)		(352)	(385)	(1,819)	(1,077)	(2,176)	(1,701)
Net income (loss)		1,413	1,687	7,303	4,493	9,016	7,096
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		(134)	(126)	(396)	(371)	(523)	(490)
Income tax expense (benefit)		49	165	174	293	220	335
Net income (loss)		(85)	39	(222)	(78)	(303)	(155)
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		$ (134)	$ (126)	$ (396)	$ (371)	$ (523)	$ (490)

[1] These items are included in the computation of net periodic benefit cost. See Note I, Employee Benefits, for additional information.