Investments (Fixed maturities by investment rating) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fixed Maturities
Fair value	$ 5,576,341	$ 5,510,331
Percentage of total	100.00%	100.00%
Managed investments
Fixed Maturities
Fair value	$ 5,419,950	$ 5,323,952
Percentage of total	97.20%	96.60%
Managed investments | Total investment grade fixed maturities
Fixed Maturities
Fair value	$ 4,764,325	$ 4,659,892
Percentage of total	85.40%	84.60%
Managed investments | Total investment grade fixed maturities | AAA
Fixed Maturities
Fair value	$ 2,417,068	$ 2,367,642
Percentage of total	43.30%	43.00%
Managed investments | Total investment grade fixed maturities | AA
Fixed Maturities
Fair value	$ 526,729	$ 569,386
Percentage of total	9.50%	10.30%
Managed investments | Total investment grade fixed maturities | A
Fixed Maturities
Fair value	$ 1,084,128	$ 1,031,326
Percentage of total	19.40%	18.70%
Managed investments | Total investment grade fixed maturities | BBB
Fixed Maturities
Fair value	$ 736,400	$ 691,538
Percentage of total	13.20%	12.60%
Managed investments | Total non-investment grade fixed maturities
Fixed Maturities
Fair value	$ 655,625	$ 664,060
Percentage of total	11.80%	12.00%
Managed investments | Total non-investment grade fixed maturities | BB
Fixed Maturities
Fair value	$ 228,798	$ 235,724
Percentage of total	4.10%	4.30%
Managed investments | Total non-investment grade fixed maturities | B
Fixed Maturities
Fair value	$ 182,261	$ 179,069
Percentage of total	3.30%	3.20%
Managed investments | Total non-investment grade fixed maturities | CCC
Fixed Maturities
Fair value	$ 9,561	$ 5,706
Percentage of total	0.20%	0.10%
Managed investments | Total non-investment grade fixed maturities | CC
Fixed Maturities
Fair value	$ 0	$ 1,015
Percentage of total	0.00%	0.00%
Managed investments | Total non-investment grade fixed maturities | NR
Fixed Maturities
Fair value	$ 235,005	$ 242,546
Percentage of total	4.20%	4.40%
Non-managed investments
Fixed Maturities
Fair value	$ 156,391	$ 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	$ 156,391	$ 184,468
Percentage of total	2.80%	3.40%
Non-managed investments | Total non-investment grade fixed maturities | BB
Fixed Maturities
Fair value	$ 31,052	$ 70,962
Percentage of total	0.50%	1.30%
Non-managed investments | Total non-investment grade fixed maturities | B
Fixed Maturities
Fair value	$ 4,922	$ 30,698
Percentage of total	0.10%	0.60%
Non-managed investments | Total non-investment grade fixed maturities | NR
Fixed Maturities
Fair value	$ 120,417	$ 82,808
Percentage of total	2.20%	1.50%