Investments (Fixed Maturities and Equity Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	$ 7,120,844	$ 6,775,806
Amortized Cost	59,599	54,609
Corporate securities	7,412,357	6,840,932
Fair Value	57,578	51,973
Net unrealized gains on impaired securities relating to changes in value of securities subsequent to the impairment measurement date	9,000	9,000
Fixed maturities, available-for-sale
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	7,120,844	6,775,806
Gross Unrealized Gains	360,764	217,935
Gross Unrealized Losses	69,251	152,809
Corporate securities	7,412,357	6,840,932
OTTI in AOCI	(3,975)	(3,963)
Fixed maturities, available-for-sale | U.S. Treasury securities and obligations of U.S. government authorities and agencies
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	165,404	87,107
Gross Unrealized Gains	11,764	7,170
Gross Unrealized Losses	1	228
Corporate securities	177,167	94,049
OTTI in AOCI	0	0
Fixed maturities, available-for-sale | Obligations of U.S. states and their political subdivisions
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	631,330	602,508
Gross Unrealized Gains	39,057	24,219
Gross Unrealized Losses	184	1,958
Corporate securities	670,203	624,769
OTTI in AOCI	0	0
Fixed maturities, available-for-sale | Foreign government bonds
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	95,589	70,107
Gross Unrealized Gains	4,019	3,094
Gross Unrealized Losses	640	2,791
Corporate securities	98,968	70,410
OTTI in AOCI	0	0
Fixed maturities, available-for-sale | Public utilities
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	839,673	790,038
Gross Unrealized Gains	57,342	30,862
Gross Unrealized Losses	4,932	18,402
Corporate securities	892,083	802,498
OTTI in AOCI	0	0
Fixed maturities, available-for-sale | Redeemable preferred stock
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	17,345	5,316
Gross Unrealized Gains	2,079	1,530
Gross Unrealized Losses	156	145
Corporate securities	19,268	6,701
OTTI in AOCI	0	0
Fixed maturities, available-for-sale | All other U.S. public corporate securities
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	2,280,085	2,138,358
Gross Unrealized Gains	139,206	81,905
Gross Unrealized Losses	24,873	61,142
Corporate securities	2,394,418	2,159,121
OTTI in AOCI	(191)	(217)
Fixed maturities, available-for-sale | All other U.S. private corporate securities
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	1,121,831	1,085,345
Gross Unrealized Gains	38,499	26,299
Gross Unrealized Losses	8,461	13,963
Corporate securities	1,151,869	1,097,681
OTTI in AOCI	(224)	0
Fixed maturities, available-for-sale | All other foreign public corporate securities
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	268,944	270,063
Gross Unrealized Gains	12,143	8,230
Gross Unrealized Losses	3,353	6,508
Corporate securities	277,734	271,785
OTTI in AOCI	0	0
Fixed maturities, available-for-sale | All other foreign private corporate securities
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	784,955	784,283
Gross Unrealized Gains	18,316	9,933
Gross Unrealized Losses	23,873	42,528
Corporate securities	779,398	751,688
OTTI in AOCI	0	0
Fixed maturities, available-for-sale | Asset-backed securities
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	350,399	431,578
Gross Unrealized Gains	5,866	6,203
Gross Unrealized Losses	2,679	2,650
Corporate securities	353,586	435,131
OTTI in AOCI	(2,903)	(3,056)
Fixed maturities, available-for-sale | Commercial mortgage-backed securities
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	455,185	396,160
Gross Unrealized Gains	23,655	10,614
Gross Unrealized Losses	62	2,429
Corporate securities	478,778	404,345
OTTI in AOCI	0	0
Fixed maturities, available-for-sale | Residential mortgage-backed securities
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	110,104	114,943
Gross Unrealized Gains	8,818	7,876
Gross Unrealized Losses	37	65
Corporate securities	118,885	122,754
OTTI in AOCI	(657)	(690)
Equity securities, available-for-sale
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	59,599	54,609
Gross Unrealized Gains	840	423
Gross Unrealized Losses	2,861	3,059
Fair Value	57,578	51,973
Equity securities, available-for-sale | Public utilities
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	66	66
Gross Unrealized Gains	2	2
Gross Unrealized Losses	25	29
Fair Value	43	39
Equity securities, available-for-sale | Industrial, miscellaneous & other
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	0	0
Gross Unrealized Gains	150	165
Gross Unrealized Losses	0	0
Fair Value	150	165
Equity securities, available-for-sale | Mutual funds
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	59,533	54,543
Gross Unrealized Gains	688	256
Gross Unrealized Losses	2,836	3,030
Fair Value	57,385	51,769
Equity securities, available-for-sale | Non-redeemable preferred stocks
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 0	$ 0