Investments - Other than temporary impairments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Other Than Temporary Impairments [Line Items]
OTTI on investments, net of amounts transferred to other comprehensive income	$ 13,345	$ 78,558	$ 82,960
Fixed maturities | Credit loss impairments
Schedule of Other Than Temporary Impairments [Line Items]
OTTI on investments, net of amounts transferred to other comprehensive income	1,177	15,100	11,928
Fixed maturities | No longer intend to hold to recovery
Schedule of Other Than Temporary Impairments [Line Items]
OTTI on investments, net of amounts transferred to other comprehensive income	0	3,425	18,759
Equities and other investments | No longer intend to hold to recovery
Schedule of Other Than Temporary Impairments [Line Items]
OTTI on investments, net of amounts transferred to other comprehensive income	0	14,849	6,858
Equities and other investments | Impaired more than 11 months or more than 50%
Schedule of Other Than Temporary Impairments [Line Items]
OTTI on investments, net of amounts transferred to other comprehensive income	5,766	15,277	21,756
Exclude Life Funds Withheld Assets | Foreign exchange
Schedule of Other Than Temporary Impairments [Line Items]
OTTI on investments, net of amounts transferred to other comprehensive income	$ 6,402	$ 29,907	$ 23,659