Investments (Fixed maturities by investment rating) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fixed Maturities
Estimated fair value	$ 5,510,331	$ 5,545,231
Percentage of total	100.00%	100.00%
Managed investments
Fixed Maturities
Estimated fair value	$ 5,323,952	$ 5,457,067
Percentage of total	96.60%	98.40%
Managed investments | Investment Grade
Fixed Maturities
Estimated fair value	$ 4,659,892	$ 4,933,764
Percentage of total	84.60%	89.00%
Managed investments | Investment Grade | AAA [Member]
Fixed Maturities
Estimated fair value	$ 2,367,642	$ 2,494,239
Percentage of total	43.00%	45.00%
Managed investments | Investment Grade | AA [Member]
Fixed Maturities
Estimated fair value	$ 569,386	$ 848,226
Percentage of total	10.30%	15.30%
Managed investments | Investment Grade | A [Member]
Fixed Maturities
Estimated fair value	$ 1,031,326	$ 1,086,091
Percentage of total	18.70%	19.60%
Managed investments | Investment Grade | BBB [Member]
Fixed Maturities
Estimated fair value	$ 691,538	$ 505,208
Percentage of total	12.60%	9.10%
Managed investments | Non-Investment Grade
Fixed Maturities
Estimated fair value	$ 664,060	$ 523,303
Percentage of total	12.00%	9.40%
Managed investments | Non-Investment Grade | BB [Member]
Fixed Maturities
Estimated fair value	$ 235,724	$ 338,108
Percentage of total	4.30%	6.10%
Managed investments | Non-Investment Grade | B [Member]
Fixed Maturities
Estimated fair value	$ 179,069	$ 113,863
Percentage of total	3.20%	2.00%
Managed investments | Non-Investment Grade | CCC [Member]
Fixed Maturities
Estimated fair value	$ 5,706	$ 12,733
Percentage of total	0.10%	0.20%
Managed investments | Non-Investment Grade | CC [Member]
Fixed Maturities
Estimated fair value	$ 1,015	$ 3,926
Percentage of total	0.00%	0.10%
Managed investments | Non-Investment Grade | C [Member]
Fixed Maturities
Estimated fair value	$ 0	$ 1,344
Percentage of total	0.00%	0.00%
Managed investments | Non-Investment Grade | NR [Member]
Fixed Maturities
Estimated fair value	$ 242,546	$ 53,329
Percentage of total	4.40%	1.00%
Non-managed investments
Fixed Maturities
Estimated fair value	$ 186,379	$ 88,164
Percentage of total	3.40%	1.60%
Non-managed investments | Investment Grade | BBB [Member]
Fixed Maturities
Estimated fair value	$ 1,911	$ 0
Percentage of total	0.00%	0.00%
Non-managed investments | Non-Investment Grade
Fixed Maturities
Estimated fair value	$ 184,468	$ 88,164
Percentage of total	3.40%	1.60%
Non-managed investments | Non-Investment Grade | BB [Member]
Fixed Maturities
Estimated fair value	$ 70,962	$ 24,864
Percentage of total	1.30%	0.40%
Non-managed investments | Non-Investment Grade | B [Member]
Fixed Maturities
Estimated fair value	$ 30,698	$ 31,377
Percentage of total	0.60%	0.60%
Non-managed investments | Non-Investment Grade | NR [Member]
Fixed Maturities
Estimated fair value	$ 82,808	$ 31,923
Percentage of total	1.50%	0.60%