Investment Information - Narrative (Details) $ in Thousands	3 Months Ended		6 Months Ended			12 Months Ended
	Jun. 30, 2018 USD ($) lots	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) lots		Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) lots	Mar. 31, 2018 companies
Narrative items:
Number of Limited Partnerships | companies							3
Investable assets	$ 21,820,000		$ 21,820,000
Equity in net income (loss) of investment funds accounted for using the equity method	8,472	$ 32,706	36,541		$ 80,794
OTTI unrealized (losses) gains at current fair value	$ 200		$ 200			$ 300
Continuous unrealized loss, qualitative disclosures:
Number of positions in an unrealized loss position (lots) | lots	5,440		5,440			3,830
Total number of positions (lots) | lots	7,450		7,450			7,450
Largest single loss	$ 1,700		$ 1,700			$ 1,300
OTTI unrealized losses (gains)			$ 84	[1]		$ 88
Minimum
Narrative items:
Time lag for reporting			1 month
Maximum
Narrative items:
Time lag for reporting			3 months
Underwriting segments
Narrative items:
Investable assets	19,170,000		$ 19,170,000
Equity in net income (loss) of investment funds accounted for using the equity method	8,472	32,706	36,541		80,794
Other
Narrative items:
Investable assets	2,650,000		2,650,000
Equity in net income (loss) of investment funds accounted for using the equity method	$ 0	$ 0	$ 0		$ 0

[1] Represents the total other-than-temporary impairments (“OTTI”) recognized in accumulated other comprehensive income (“AOCI”). It does not include the change in fair value subsequent to the impairment measurement date. At June 30, 2018, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $0.2 million, compared to a net unrealized gain of $0.3 million at December 31, 2017.