Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year		$ 973,373
Cumulative effect of change in accounting principle			$ (191,244)	$ 0
Change in accumulated other comprehensive income before reclassifications	$ (12,157)	(51,274)
Reclassification adjustments from accumulated other comprehensive income (loss)	1,704	4,453
Balance, end of period	884,216	884,216
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	(15,370)	214,865
Cumulative effect of change in accounting principle			(191,244)
Change in accumulated other comprehensive income before reclassifications	(12,157)	(51,274)
Reclassification adjustments from accumulated other comprehensive income (loss)	392	518
Balance, end of period	(27,135)	(27,135)
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	(43,928)	(46,551)
Cumulative effect of change in accounting principle			0
Change in accumulated other comprehensive income before reclassifications	0	0
Reclassification adjustments from accumulated other comprehensive income (loss)	1,312	3,935
Balance, end of period	(42,616)	(42,616)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	(59,298)	168,314
Cumulative effect of change in accounting principle			$ (191,244)
Balance, end of period	$ (69,751)	$ (69,751)