Investment Information - Narrative (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($) lots	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) lots	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) lots
Narrative items:
Investable assets	$ 23,550,000		$ 23,550,000
Equity in net income of investment funds accounted for using the equity method	32,536	$ 8,472	79,403	$ 36,541
OTTI unrealized (losses) gains at current fair value	$ (10)		$ (10)		$ (40)
Continuous unrealized loss, qualitative disclosures:
Number of positions in an unrealized loss position (lots) | lots	1,690		1,690		5,870
Total number of positions (lots) | lots	8,730		8,730		8,450
Largest single loss	$ 3,100		$ 3,100		$ 2,600
Equity securities, at fair value	670,943		670,943		338,899
OTTI unrealized losses (gains)			$ 6		$ 75
Minimum
Narrative items:
Time lag for reporting			1 month
Maximum
Narrative items:
Time lag for reporting			3 months
Underwriting segments
Narrative items:
Investable assets	20,780,000		$ 20,780,000
Equity in net income of investment funds accounted for using the equity method	32,536	8,472	79,403	36,541
Other
Narrative items:
Investable assets	2,770,000		2,770,000
Equity in net income of investment funds accounted for using the equity method	$ 0	$ 0	$ 0	$ 0