Investments (Fixed maturities by investment rating) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Fixed Maturities
Fair value	$ 5,551,586	$ 5,510,331
Percentage of total	100.00%	100.00%
Managed investments
Fixed Maturities
Fair value	$ 5,393,525	$ 5,323,952
Percentage of total	97.20%	96.60%
Managed investments | Total investment grade fixed maturities
Fixed Maturities
Fair value	$ 4,742,690	$ 4,659,892
Percentage of total	85.50%	84.60%
Managed investments | Total investment grade fixed maturities | AAA
Fixed Maturities
Fair value	$ 2,446,839	$ 2,367,642
Percentage of total	44.10%	43.00%
Managed investments | Total investment grade fixed maturities | AA
Fixed Maturities
Fair value	$ 508,086	$ 569,386
Percentage of total	9.20%	10.30%
Managed investments | Total investment grade fixed maturities | A
Fixed Maturities
Fair value	$ 1,045,965	$ 1,031,326
Percentage of total	18.80%	18.70%
Managed investments | Total investment grade fixed maturities | BBB
Fixed Maturities
Fair value	$ 741,800	$ 691,538
Percentage of total	13.40%	12.60%
Managed investments | Total non-investment grade fixed maturities
Fixed Maturities
Fair value	$ 650,835	$ 664,060
Percentage of total	11.70%	12.00%
Managed investments | Total non-investment grade fixed maturities | BB
Fixed Maturities
Fair value	$ 227,436	$ 235,724
Percentage of total	4.00%	4.30%
Managed investments | Total non-investment grade fixed maturities | B
Fixed Maturities
Fair value	$ 183,127	$ 179,069
Percentage of total	3.30%	3.20%
Managed investments | Total non-investment grade fixed maturities | CCC
Fixed Maturities
Fair value	$ 8,331	$ 5,706
Percentage of total	0.20%	0.10%
Managed investments | Total non-investment grade fixed maturities | CC
Fixed Maturities
Fair value	$ 113	$ 1,015
Percentage of total	0.00%	0.00%
Managed investments | Total non-investment grade fixed maturities | NR
Fixed Maturities
Fair value	$ 231,828	$ 242,546
Percentage of total	4.20%	4.40%
Non-managed investments
Fixed Maturities
Fair value	$ 158,061	$ 186,379
Percentage of total	2.80%	3.40%
Non-managed investments | Total investment grade fixed maturities
Fixed Maturities
Fair value	$ 0	$ 1,911
Percentage of total	0.00%	0.00%
Non-managed investments | Total investment grade fixed maturities | BBB
Fixed Maturities
Fair value	$ 0	$ 1,911
Percentage of total	0.00%	0.00%
Non-managed investments | Total non-investment grade fixed maturities
Fixed Maturities
Fair value	$ 158,061	$ 184,468
Percentage of total	2.80%	3.40%
Non-managed investments | Total non-investment grade fixed maturities | BB
Fixed Maturities
Fair value	$ 41,412	$ 70,962
Percentage of total	0.70%	1.30%
Non-managed investments | Total non-investment grade fixed maturities | B
Fixed Maturities
Fair value	$ 6,161	$ 30,698
Percentage of total	0.10%	0.60%
Non-managed investments | Total non-investment grade fixed maturities | NR
Fixed Maturities
Fair value	$ 110,488	$ 82,808
Percentage of total	2.00%	1.50%