Investment Information - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) lots	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) lots	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) lots
Narrative items:
Investable assets	$ 24,300,000		$ 24,300,000
Equity in net income of investment funds accounted for using the equity method	17,130	$ 15,982	96,533	$ 52,523
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	1,980		1,980		5,870
Total number of positions (lots) | lots	9,260		9,260		8,450
Largest single loss	$ 3,000		$ 3,000		$ 2,600
Equity securities, at fair value	550,485		550,485		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	21,570,000		$ 21,570,000
Equity in net income of investment funds accounted for using the equity method	17,130	15,982	96,533	52,523
Other
Narrative items:
Investable assets	2,730,000		2,730,000
Equity in net income of investment funds accounted for using the equity method	$ 0	$ 0	$ 0	$ 0