Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	$ 2,171,503	$ 626,788
Less than 12 months Gross Unrealized Losses	88,804	15,855
Twelve months or more Fair Value	129,514	455,822
Twelve months or more Gross Unrealized Losses	9,798	11,695
Total Fair Value	2,301,017	1,082,610
Total Gross Unrealized Losses	98,602	27,550
U.S. Treasury securities and obligations of U.S. government authorities and agencies
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	6,557	994
Less than 12 months Gross Unrealized Losses	281	1
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	6,557	994
Total Gross Unrealized Losses	281	1
Obligations of U.S. states and their political subdivisions
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	320,836	9,852
Less than 12 months Gross Unrealized Losses	8,318	125
Twelve months or more Fair Value	0	2,886
Twelve months or more Gross Unrealized Losses	0	62
Total Fair Value	320,836	12,738
Total Gross Unrealized Losses	8,318	187
Foreign government bonds
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	41,118	2,246
Less than 12 months Gross Unrealized Losses	1,570	14
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	41,118	2,246
Total Gross Unrealized Losses	1,570	14
Public utilities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	240,614	30,974
Less than 12 months Gross Unrealized Losses	13,963	1,618
Twelve months or more Fair Value	11,784	45,756
Twelve months or more Gross Unrealized Losses	1,667	1,670
Total Fair Value	252,398	76,730
Total Gross Unrealized Losses	15,630	3,288
All other corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	1,220,185	356,348
Less than 12 months Gross Unrealized Losses	62,518	13,194
Twelve months or more Fair Value	75,819	260,985
Twelve months or more Gross Unrealized Losses	7,768	7,763
Total Fair Value	1,296,004	617,333
Total Gross Unrealized Losses	70,286	20,957
Asset-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	204,321	209,774
Less than 12 months Gross Unrealized Losses	415	737
Twelve months or more Fair Value	35,726	54,711
Twelve months or more Gross Unrealized Losses	274	473
Total Fair Value	240,047	264,485
Total Gross Unrealized Losses	689	1,210
Commercial mortgage-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	112,489	15,824
Less than 12 months Gross Unrealized Losses	1,671	155
Twelve months or more Fair Value	4,440	87,606
Twelve months or more Gross Unrealized Losses	80	1,713
Total Fair Value	116,929	103,430
Total Gross Unrealized Losses	1,751	1,868
Residential mortgage-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	25,383	776
Less than 12 months Gross Unrealized Losses	68	11
Twelve months or more Fair Value	1,745	3,878
Twelve months or more Gross Unrealized Losses	9	14
Total Fair Value	27,128	4,654
Total Gross Unrealized Losses	77	25
Equity securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	17,194	14,706
Less than 12 months Gross Unrealized Losses	841	295
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	17,194	14,706
Total Gross Unrealized Losses	$ 841	$ 295