Investments (Fixed maturities by investment rating) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fixed Maturities
Estimated fair value	$ 5,543,030	$ 5,510,331
Percentage of total	100.00%	100.00%
Managed investments
Fixed Maturities
Estimated fair value	$ 5,384,699	$ 5,323,952
Percentage of total	97.10%	96.60%
Managed investments | Investment Grade
Fixed Maturities
Estimated fair value	$ 4,766,696	$ 4,659,892
Percentage of total	86.00%	84.60%
Managed investments | Investment Grade | AAA [Member]
Fixed Maturities
Estimated fair value	$ 2,405,597	$ 2,367,642
Percentage of total	43.40%	43.00%
Managed investments | Investment Grade | AA [Member]
Fixed Maturities
Estimated fair value	$ 538,289	$ 569,386
Percentage of total	9.70%	10.30%
Managed investments | Investment Grade | A [Member]
Fixed Maturities
Estimated fair value	$ 1,081,949	$ 1,031,326
Percentage of total	19.50%	18.70%
Managed investments | Investment Grade | BBB [Member]
Fixed Maturities
Estimated fair value	$ 740,861	$ 691,538
Percentage of total	13.40%	12.60%
Managed investments | Non-Investment Grade
Fixed Maturities
Estimated fair value	$ 618,003	$ 664,060
Percentage of total	11.10%	12.00%
Managed investments | Non-Investment Grade | BB [Member]
Fixed Maturities
Estimated fair value	$ 213,568	$ 235,724
Percentage of total	3.90%	4.30%
Managed investments | Non-Investment Grade | B [Member]
Fixed Maturities
Estimated fair value	$ 177,737	$ 179,069
Percentage of total	3.20%	3.20%
Managed investments | Non-Investment Grade | CCC [Member]
Fixed Maturities
Estimated fair value	$ 13,371	$ 5,706
Percentage of total	0.20%	0.10%
Managed investments | Non-Investment Grade | CC [Member]
Fixed Maturities
Estimated fair value	$ 0	$ 1,015
Percentage of total	0.00%	0.00%
Managed investments | Non-Investment Grade | NR [Member]
Fixed Maturities
Estimated fair value	$ 213,327	$ 242,546
Percentage of total	3.80%	4.40%
Non-managed investments
Fixed Maturities
Estimated fair value	$ 158,331	$ 186,379
Percentage of total	2.90%	3.40%
Non-managed investments | Investment Grade
Fixed Maturities
Estimated fair value	$ 0	$ 1,911
Percentage of total	0.00%	0.00%
Non-managed investments | Investment Grade | BBB [Member]
Fixed Maturities
Estimated fair value	$ 0	$ 1,911
Percentage of total	0.00%	0.00%
Non-managed investments | Non-Investment Grade
Fixed Maturities
Estimated fair value	$ 158,331	$ 184,468
Percentage of total	2.90%	3.40%
Non-managed investments | Non-Investment Grade | BB [Member]
Fixed Maturities
Estimated fair value	$ 29,731	$ 70,962
Percentage of total	0.60%	1.30%
Non-managed investments | Non-Investment Grade | B [Member]
Fixed Maturities
Estimated fair value	$ 4,524	$ 30,698
Percentage of total	0.10%	0.60%
Non-managed investments | Non-Investment Grade | NR [Member]
Fixed Maturities
Estimated fair value	$ 124,076	$ 82,808
Percentage of total	2.20%	1.50%