Investments (Fixed Maturities and Equity Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	$ 5,051,015	$ 6,775,806
Amortized Cost	22,408	54,609
Corporate securities	5,387,062	6,840,932
Fair Value	23,001	51,973
Net unrealized gains on impaired securities relating to changes in value of securities subsequent to the impairment measurement date	8,000	9,000
Fixed maturities, available-for-sale
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	5,051,015	6,775,806
Gross Unrealized Gains	372,917	217,935
Gross Unrealized Losses	36,870	152,809
Corporate securities	5,387,062	6,840,932
OTTI in AOCI	(3,325)	(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	97,295	87,107
Gross Unrealized Gains	9,147	7,170
Gross Unrealized Losses	0	228
Corporate securities	106,442	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	535,802	602,508
Gross Unrealized Gains	60,097	24,219
Gross Unrealized Losses	0	1,958
Corporate securities	595,899	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	84,275	70,107
Gross Unrealized Gains	4,863	3,094
Gross Unrealized Losses	184	2,791
Corporate securities	88,954	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	636,650	790,038
Gross Unrealized Gains	68,052	30,862
Gross Unrealized Losses	2,802	18,402
Corporate securities	701,900	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	4,818	5,316
Gross Unrealized Gains	975	1,530
Gross Unrealized Losses	200	145
Corporate securities	5,593	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	1,675,441	2,138,358
Gross Unrealized Gains	145,623	81,905
Gross Unrealized Losses	8,071	61,142
Corporate securities	1,812,993	2,159,121
OTTI in AOCI	(241)	(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	670,299	1,085,345
Gross Unrealized Gains	30,256	26,299
Gross Unrealized Losses	4,473	13,963
Corporate securities	696,082	1,097,681
OTTI in AOCI	(230)	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	183,326	270,063
Gross Unrealized Gains	12,385	8,230
Gross Unrealized Losses	1,613	6,508
Corporate securities	194,098	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	571,278	784,283
Gross Unrealized Gains	14,102	9,933
Gross Unrealized Losses	18,075	42,528
Corporate securities	567,305	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	196,034	431,578
Gross Unrealized Gains	4,855	6,203
Gross Unrealized Losses	1,385	2,650
Corporate securities	199,504	435,131
OTTI in AOCI	(2,557)	(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	310,468	396,160
Gross Unrealized Gains	16,861	10,614
Gross Unrealized Losses	37	2,429
Corporate securities	327,292	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	85,329	114,943
Gross Unrealized Gains	5,701	7,876
Gross Unrealized Losses	30	65
Corporate securities	91,000	122,754
OTTI in AOCI	(297)	(690)
Equity securities, available-for-sale
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	22,408	54,609
Gross Unrealized Gains	912	423
Gross Unrealized Losses	319	3,059
Fair Value	23,001	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	3	2
Gross Unrealized Losses	19	29
Fair Value	50	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	191	165
Gross Unrealized Losses	0	0
Fair Value	191	165
Equity securities, available-for-sale | Mutual funds
Summary of Investments Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	22,342	54,543
Gross Unrealized Gains	718	256
Gross Unrealized Losses	300	3,030
Fair Value	22,760	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