Fair Value Measurements - Additional information about assets and liabilities measured at fair value on a recurring basis (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Level 3 assets
Balance, beginning of period	$ 246,544	$ 251,596
Realized gains (losses)	493	(5,513)
Movement in unrealized gains (losses)	5,632	6,496
Purchases and issuances	30,667	16,675
Sales	(13,016)	(10,284)
Settlements	(11,633)	(5,986)
Net transfers into (out of) Level 3	6,851	(552)
Balance, end of period	265,538	252,432
Movement in total gains (losses) above relating to instruments still held at the reporting date	5,658	5,768
Derivative contracts - net
Balance, beginning of period	22	2
Realized gains (losses)	0	0
Movement in unrealized gains (losses)	312	161
Purchases and issuances	0	0
Sales	0	0
Settlements	0	0
Net transfers into (ouf ot) Level 3	0	0
Balance, end of period	334	163
Movement in total gains (losses) above relating to instruments still held at the reporting date	312	163
U.S. Government and Government Agencies
Level 3 assets
Balance, beginning of period	18,290	24,751
Realized gains (losses)	(354)	(19)
Movement in unrealized gains (losses)	114	(355)
Purchases and issuances	0	0
Sales	(13,016)	0
Settlements	0	(311)
Net transfers into (out of) Level 3	0	(4,758)
Balance, end of period	5,034	19,308
Movement in total gains (losses) above relating to instruments still held at the reporting date	(36)	(373)
Corporate
Level 3 assets
Balance, beginning of period	1,292	20,085
Realized gains (losses)	0	0
Movement in unrealized gains (losses)	43	11
Purchases and issuances	0	0
Sales	0	(10,284)
Settlements	(101)	(31)
Net transfers into (out of) Level 3	894	118
Balance, end of period	2,128	9,899
Movement in total gains (losses) above relating to instruments still held at the reporting date	44	11
Residential mortgage-backed securities
Level 3 assets
Balance, beginning of period	0	0
Realized gains (losses)	0	17
Movement in unrealized gains (losses)	0	(526)
Purchases and issuances	980	0
Sales	0	0
Settlements	0	(173)
Net transfers into (out of) Level 3	5,957	4,063
Balance, end of period	6,937	3,381
Movement in total gains (losses) above relating to instruments still held at the reporting date	0	(509)
Commercial mortgage-backed securities
Level 3 assets
Balance, beginning of period		99
Realized gains (losses)		0
Movement in unrealized gains (losses)		2
Purchases and issuances		0
Sales		0
Settlements		0
Net transfers into (out of) Level 3		0
Balance, end of period		101
Movement in total gains (losses) above relating to instruments still held at the reporting date		2
Other asset-backed securities
Level 3 assets
Balance, beginning of period	5,232	1,131
Realized gains (losses)	0	30
Movement in unrealized gains (losses)	(28)	101
Purchases and issuances	15,407	0
Sales	0	0
Settlements	(874)	(46)
Net transfers into (out of) Level 3	0	2,734
Balance, end of period	19,737	3,950
Movement in total gains (losses) above relating to instruments still held at the reporting date	0	131
Short-term investments
Level 3 assets
Balance, beginning of period		0
Realized gains (losses)		0
Movement in unrealized gains (losses)		0
Purchases and issuances		11,693
Sales		0
Settlements		(28)
Net transfers into (out of) Level 3		0
Balance, end of period		11,665
Movement in total gains (losses) above relating to instruments still held at the reporting date		0
Other investments
Level 3 assets
Balance, beginning of period	221,708	205,528
Realized gains (losses)	847	(5,541)
Movement in unrealized gains (losses)	5,191	7,102
Purchases and issuances	14,280	4,982
Sales	0	0
Settlements	(10,658)	(5,397)
Net transfers into (out of) Level 3	0	(2,709)
Balance, end of period	231,368	203,965
Movement in total gains (losses) above relating to instruments still held at the reporting date	$ 5,338	$ 6,343