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, 2017	Mar. 31, 2016
Level 3 assets
Balance, beginning of period	$ 251,596	$ 426,037
Realized gains (losses)	(5,513)	6,910
Movement in unrealized gains (losses)	6,496	281
Purchases and issuances	16,675	13,948
Sales	(10,284)	0
Settlements	(5,986)	(43,656)
Net transfers into (out of) Level 3	(552)	5,209
Balance, end of period	252,432	408,729
Movement in total gains (losses) above relating to instruments still held at the reporting date	5,768	7,190
Derivative contracts - net
Balance, beginning of period	2	(9,791)
Realized gains (losses)	0	0
Movement in unrealized gains (losses)	161	(61)
Purchases and issuances	0	0
Sales	0	0
Settlements	0	0
Net transfers into (ouf ot) Level 3	0	0
Balance, end of period	163	(9,852)
Movement in total gains (losses) above relating to instruments still held at the reporting date	163	(61)
U.S. Government and Government Agencies
Level 3 assets
Balance, beginning of period	24,751	45,063
Realized gains (losses)	(19)	(37)
Movement in unrealized gains (losses)	(355)	593
Purchases and issuances	0	0
Sales	0	0
Settlements	(311)	(679)
Net transfers into (out of) Level 3	(4,758)	0
Balance, end of period	19,308	44,940
Movement in total gains (losses) above relating to instruments still held at the reporting date	(373)	554
Corporate
Level 3 assets
Balance, beginning of period	20,085	53,873
Realized gains (losses)	0	53
Movement in unrealized gains (losses)	11	(40)
Purchases and issuances	0	169
Sales	(10,284)	0
Settlements	(31)	(6)
Net transfers into (out of) Level 3	118	(10,285)
Balance, end of period	9,899	43,764
Movement in total gains (losses) above relating to instruments still held at the reporting date	11	14
Residential mortgage-backed securities
Level 3 assets
Balance, beginning of period	0	3,077
Realized gains (losses)	17	(3)
Movement in unrealized gains (losses)	(526)	7
Purchases and issuances	0	0
Sales	0	0
Settlements	(173)	(228)
Net transfers into (out of) Level 3	4,063	0
Balance, end of period	3,381	2,853
Movement in total gains (losses) above relating to instruments still held at the reporting date	(509)	4
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	1,131	50,265
Realized gains (losses)	30	1,090
Movement in unrealized gains (losses)	101	2,499
Purchases and issuances	0	0
Sales	0	0
Settlements	(46)	(27,925)
Net transfers into (out of) Level 3	2,734	0
Balance, end of period	3,950	25,929
Movement in total gains (losses) above relating to instruments still held at the reporting date	131	3,589
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	205,528	283,550
Realized gains (losses)	(5,541)	5,807
Movement in unrealized gains (losses)	7,102	(2,717)
Purchases and issuances	4,982	13,779
Sales	0	0
Settlements	(5,397)	(14,818)
Net transfers into (out of) Level 3	(2,709)	15,494
Balance, end of period	203,965	301,095
Movement in total gains (losses) above relating to instruments still held at the reporting date	$ 6,343	$ 3,090