Accumulated Other Comprehensive Income Amounts Reclassified (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income (Loss) before income tax		$ 117,770	$ 81,340	$ 142,535	$ 116,359	$ 142,989	$ 122,790
Income tax expense		(43,134)	(24,600)	(51,423)	(35,089)	(51,229)	(37,279)
Net (Income) Loss		74,636	56,740	91,112	81,270	91,760	85,511
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]	1,663	1,606	4,993	4,931	6,636	6,996
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]	(1,087)	(1,966)	(3,532)	(6,466)	(5,688)	(8,081)
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1]	576	(360)	1,461	(1,535)	948	(1,085)
Reclassification from AOCI, Current Period, Tax		228	134	6	756	60	597
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	[1]	804	(226)	1,467	(779)	1,008	(488)
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		1,687	3,472	4,493	6,334	7,096	9,219
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		39	(72)	(78)	(187)	(155)	(259)
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		2,530	3,174	5,882	5,368	7,949	8,472
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		2,072	4,324	5,570	7,887	8,797	11,446
Income (Loss) before income tax		2,072	4,324	5,570	7,887	8,797	11,446
Income tax expense		(385)	(852)	(1,077)	(1,553)	(1,701)	(2,227)
Net (Income) Loss		1,687	3,472	4,493	6,334	7,096	9,219
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 tax		(126)	(118)	(371)	(348)	(490)	(460)
Income tax expense		165	46	293	161	335	201
Net (Income) Loss		39	(72)	(78)	(187)	(155)	(259)
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		$ (126)	$ (118)	$ (371)	$ (348)	$ (490)	$ (460)

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