Accumulated Other Comprehensive Income Amounts Reclassified (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income (loss) before income taxes	$ 33,399		$ 44,060		$ 41,389	$ 34,042	$ 118,030	$ 133,738
Income tax expense (benefit)	(7,273)		(10,765)		(9,174)	(7,713)	(27,829)	(41,225)
Net income	26,126	$ 6,089	33,295	$ (6,966)	32,215	26,329	90,201	92,513
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	2,184		2,414		4,370	4,830	9,197	9,658
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	(865)		(1,575)		(1,708)	(3,150)	(4,945)	(6,538)
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	1,319		839		2,662	1,680	4,252	3,120
Reclassification from AOCI, Current Period, Tax	(296)		(190)		(561)	(346)	(941)	(960)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1,023		649		2,101	1,334	3,311	2,160
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(111)		(115)		(773)	(519)	(1,390)	2,115
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	(117)		(109)		(217)	(198)	(442)	(370)
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	795		425		1,111	617	1,479	3,905
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	(141)		(147)		(970)	(665)	(1,750)	2,604
Income (loss) before income taxes	(141)		(147)		(970)	(665)	(1,750)	2,604
Income tax expense (benefit)	30		32		197	146	360	(489)
Net income	(111)		(115)		(773)	(519)	(1,390)	2,115
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	(150)		(140)		(298)	(279)	(587)	(549)
Income tax expense (benefit)	33		31		81	81	145	179
Net income	(117)		(109)		(217)	(198)	(442)	(370)
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	$ (150)		$ (140)		$ (298)	$ (279)	$ (587)	$ (549)