Investment Information - Fair value option (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	$ 4,111,611	$ 4,216,237
Fixed maturities
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	1,428,503	1,642,855
Other investments
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	2,328,904	2,136,381
Short-term investments
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	217,066	297,426
Equity securities
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments accounted for using the fair value option	$ 137,138	$ 139,575