Investment Information - Narrative (Details) $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
		Jun. 30, 2017 USD ($) lots	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) lots	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) lots
Narrative items:
Investable assets		$ 21,110,000		$ 21,110,000
Income (Loss) from Equity Method Investments		32,706	$ 8,737	80,794	$ 15,392
OTTI unrealized (losses) gains at current fair value		$ 800		$ 800		$ 2,800
Continuous unrealized loss, qualitative disclosures:
Number of positions in an unrealized loss position (lots) | lots		3,150		3,150		3,540
Total number of positions (lots) | lots		7,610		7,610		7,240
Largest single loss		$ 2,500		$ 2,500		$ 4,600
OTTI unrealized losses (gains)	[1]			3,592		$ 5,831
Underwriting segments
Narrative items:
Investable assets		19,170,000		19,170,000
Income (Loss) from Equity Method Investments		32,706	8,737	80,794	15,392
Other
Narrative items:
Investable assets		1,930,000		1,930,000
Income (Loss) from Equity Method Investments		$ 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, 2017, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $0.8 million, compared to a net unrealized gain of $2.8 million at December 31, 2016.