Schedule I - Condensed Financial Information of Registrant - Condensed Statements of Comprehensive Income (Loss), Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Statement of Income Captions [Line Items]
Net unrealized holding gains (losses) on investments arising during the period, tax provision (benefit)	$ 3,450	$ 2,262	$ (5,029)
Less: reclassification adjustment for net (gains) losses included in net income (loss), tax provision (benefit)	(124)	(2)	(6,068)
Net change in unrealized holding gains (losses) on investments, tax provision (benefit)	3,326	2,260	(11,097)
Net unrealized foreign exchange gains (losses) arising during the period, tax provision (benefit)	14,616	(3,530)	(5,174)
Less: reclassification adjustment for foreign exchange (gains) losses included in net income (loss), tax provision (benefit)	(1,086)	0	0
Net change in unrealized foreign exchange gains (losses), tax provision (benefit)	13,530	(3,530)	(5,174)
Net unrealized gains (losses) on instrument specific credit risk arising during the period, income tax provision (benefit)	(13,215)	(4,251)	0
Net change in unrealized instrument specific credit risk gains (losses), income tax provision (benefit)	(13,215)	(4,251)	0
Net unrealized gains (losses) on derivatives arising during the period, tax provision (benefit)	(593)	0	0
Net change in unrealized derivative gains (losses), tax provision (benefit)	(593)	0	0
Net pension and postretirement gains (losses) arising during the period, tax provision (benefit)	2,018	(2,516)	7,152
Less: reclassification adjustment for pension and postretirement (gains) losses included in net income (loss), tax provision (benefit)	(2,042)	(700)	(17,159)
Net change in pension liability and postretirement benefits, tax provision (benefit)	4,060	(1,816)	24,311
Parent Company
Condensed Statement of Income Captions [Line Items]
Net unrealized holding gains (losses) on investments arising during the period, tax provision (benefit)	3,450	2,262	(5,029)
Less: reclassification adjustment for net (gains) losses included in net income (loss), tax provision (benefit)	(124)	(2)	(6,068)
Net change in unrealized holding gains (losses) on investments, tax provision (benefit)	3,326	2,260	(11,097)
Net unrealized foreign exchange gains (losses) arising during the period, tax provision (benefit)	14,616	(3,530)	(5,174)
Less: reclassification adjustment for foreign exchange (gains) losses included in net income (loss), tax provision (benefit)	(1,086)	0	0
Net change in unrealized foreign exchange gains (losses), tax provision (benefit)	13,530	(3,530)	(5,174)
Net unrealized gains (losses) on instrument specific credit risk arising during the period, income tax provision (benefit)	(13,215)	(4,251)	0
Net change in unrealized instrument specific credit risk gains (losses), income tax provision (benefit)	(13,215)	(4,251)	0
Net unrealized gains (losses) on derivatives arising during the period, tax provision (benefit)	(593)	0	0
Net change in unrealized derivative gains (losses), tax provision (benefit)	(593)	0	0
Net pension and postretirement gains (losses) arising during the period, tax provision (benefit)	2,018	(2,516)	7,152
Less: reclassification adjustment for pension and postretirement (gains) losses included in net income (loss), tax provision (benefit)	(2,042)	(700)	(17,159)
Net change in pension liability and postretirement benefits, tax provision (benefit)	$ 4,060	$ (1,816)	$ 24,311