Other Comprehensive Income - Summary of Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Unrealized (losses) gains arising during the period, before U.S. income taxes	$ (6,633)	$ 7,853	$ (25,925)	$ 13,851
U.S. income taxes	155	(1,156)	699	(2,792)
Unrealized (losses) gains arising during the period, net of U.S. income taxes	(6,478)	6,697	(25,226)	11,059
Less reclassification adjustment:
Net realized investment losses (gains)	108	(12)	(93)	438
U.S. income tax expenses	(28)	1	(28)	(121)
Reclassification adjustment for investment losses (gains) realized in net income	80	(11)	(121)	317
Other comprehensive (loss) income	$ (6,558)	$ 6,708	$ (25,105)	$ 10,742

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