6. Accumulated Other Comprehensive Income 6. Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, Accumulated other comprehensive income			$ 738,794
Total other comprehensive income (loss) net of tax	$ (122,795)	$ 94,305	(78,247)	$ 193,182
Ending balance, Accumulated other comprehensive income	(18,710)		(18,710)
Accumulated Translation Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, Accumulated other comprehensive income			77,400	1,300
Other Comprehensive Income (Loss), before Reclassifications, before Tax			(77,600)	60,500
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax			0	0
Other Comprehensive Income (Loss), Tax			0	0
Total other comprehensive income (loss) net of tax			(77,600)	60,500
Ending balance, Accumulated other comprehensive income	(200)	61,800	(200)	61,800
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, Accumulated other comprehensive income			(22,300)	(18,600)
Other Comprehensive Income (Loss), before Reclassifications, before Tax			500	(2,500)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax			800	0
Other Comprehensive Income (Loss), Tax			(100)	500
Total other comprehensive income (loss) net of tax			1,200	(2,000)
Ending balance, Accumulated other comprehensive income	(21,100)	(20,600)	(21,100)	(20,600)
Accumulated Defined Benefit Plans Adjustment [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other (income) expense, net	(500)	200	(800)	0
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, Accumulated other comprehensive income			4,500	435,000
Other Comprehensive Income (Loss), before Reclassifications, before Tax			(2,100)	213,200
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax			200	0
Other Comprehensive Income (Loss), Tax			0	(78,500)
Total other comprehensive income (loss) net of tax			(1,900)	134,700
Ending balance, Accumulated other comprehensive income	2,600	569,700	2,600	569,700
Accumulated Net Unrealized Investment Gain (Loss) [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other (income) expense, net	100	200	200	0
Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, Accumulated other comprehensive income			59,600	417,700
Other Comprehensive Income (Loss), before Reclassifications, before Tax			(79,200)	271,200
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax			1,000	0
Other Comprehensive Income (Loss), Tax			(100)	(78,000)
Total other comprehensive income (loss) net of tax			(78,300)	193,200
Ending balance, Accumulated other comprehensive income	$ (18,700)	$ 610,900	$ (18,700)	$ 610,900