Accumulated Other Comprehensive Income Amounts Reclassified (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income (loss) before income taxes	$ (10,018)	$ (6,277)	$ 145,525	$ 153,378
Income tax expense (benefit)	3,052	2,288	(50,240)	(54,791)
Net income (loss)	(6,966)	(3,989)	95,285	98,587
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	2,416	2,416	9,657	8,157
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	(1,575)	(1,694)	(6,657)	(5,436)
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	841	722	3,000	2,721
Reclassification from AOCI, Current Period, Tax	(156)	(193)	(1,031)	(1,157)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	685	529	1,969	1,564
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(404)	1,756	6,364	6,849
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	(89)	(52)	(346)	(171)
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	192	2,233	7,987	8,242
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	(518)	2,191	7,917	8,443
Income (loss) before income taxes	(518)	2,191	7,917	8,443
Income tax expense (benefit)	114	(435)	(1,553)	(1,594)
Net income (loss)	(404)	1,756	6,364	6,849
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	(139)	(130)	(541)	(506)
Income tax expense (benefit)	50	78	195	335
Net income (loss)	(89)	(52)	(346)	(171)
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	$ (139)	$ (130)	$ (541)	$ (506)