Accumulated Other Comprehensive Income Amounts Reclassified (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income (loss) before income taxes	$ (45,841)	$ 7,990	$ 103,237	$ 128,691
Income tax expense (benefit)	10,632	(1,901)	(21,498)	(31,321)
Net income (loss)	(35,209)	6,089	81,739	97,370
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	1,651	2,186	8,205	9,427
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	(1,445)	(843)	(4,017)	(5,655)
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	206	1,343	4,188	3,772
Reclassification from AOCI, Current Period, Tax	(53)	(265)	(969)	(835)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	153	1,078	3,219	2,937
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0	(662)	2,375	(1,394)
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	(128)	(100)	(486)	(434)
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	25	316	5,108	1,109
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	0	(829)	3,061	(1,756)
Income (loss) before income taxes	0	(829)	3,061	(1,756)
Income tax expense (benefit)	0	167	(686)	362
Net income (loss)	0	(662)	2,375	(1,394)
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	(158)	(148)	(615)	(577)
Income tax expense (benefit)	30	48	129	143
Net income (loss)	(128)	(100)	(486)	(434)
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	$ (158)	$ (148)	$ (615)	$ (577)