Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 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	(34,219)
Reclassification adjustments from accumulated other comprehensive income (loss)	1,282
Balance, end of period	970,693
Net unrealized appreciation on investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	214,865
Cumulative effect of change in accounting principle		(191,244)
Change in accumulated other comprehensive income before reclassifications	(34,219)
Reclassification adjustments from accumulated other comprehensive income (loss)	(29)
Balance, end of period	(10,627)
Liability for underfunded employee benefit costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	(46,551)
Cumulative effect of change in accounting principle		0
Change in accumulated other comprehensive income before reclassifications	0
Reclassification adjustments from accumulated other comprehensive income (loss)	1,311
Balance, end of period	(45,240)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, beginning of year	168,314
Cumulative effect of change in accounting principle		$ (191,244)
Balance, end of period	$ (55,867)