Investments (Fixed Maturities and Equity Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Summary of Investments [Line Items]
Total fixed maturities, available-for-sale	$ 5,552,911	$ 6,775,806
Total fixed maturities, available-for-sale	5,617,549	6,840,932
Equity securities, available-for-sale, cost	16,390	54,609
Equity securities, available-for-sale, at fair value	16,756	51,973
Fixed maturities, available-for-sale:
Summary of Investments [Line Items]
Total fixed maturities, available-for-sale	5,552,911	6,775,806
Gross Unrealized Gains	167,114	217,935
Gross Unrealized Losses	102,476	152,809
Total fixed maturities, available-for-sale	5,617,549	6,840,932
OTTI in AOCI	(3,259)	(3,963)
Fixed maturities, available-for-sale: | U.S. Treasury securities and obligations of U.S. government authorities and agencies
Summary of Investments [Line Items]
Total fixed maturities, available-for-sale	154,180	87,107
Gross Unrealized Gains	6,593	7,170
Gross Unrealized Losses	33	228
Total fixed maturities, available-for-sale	160,740	94,049
OTTI in AOCI	0	0
Fixed maturities, available-for-sale: | Obligations of U.S. states and their political subdivisions
Summary of Investments [Line Items]
Total fixed maturities, available-for-sale	618,447	602,508
Gross Unrealized Gains	14,592	24,219
Gross Unrealized Losses	6,553	1,958
Total fixed maturities, available-for-sale	626,486	624,769
OTTI in AOCI	0	0
Fixed maturities, available-for-sale: | Foreign government bonds
Summary of Investments [Line Items]
Total fixed maturities, available-for-sale	111,025	70,107
Gross Unrealized Gains	2,143	3,094
Gross Unrealized Losses	4,386	2,791
Total fixed maturities, available-for-sale	108,782	70,410
OTTI in AOCI	0	0
Fixed maturities, available-for-sale: | Public utilities
Summary of Investments [Line Items]
Total fixed maturities, available-for-sale	706,536	790,038
Gross Unrealized Gains	33,950	30,862
Gross Unrealized Losses	10,519	18,402
Total fixed maturities, available-for-sale	729,967	802,498
OTTI in AOCI	0	0
Fixed maturities, available-for-sale: | Redeemable preferred stock
Summary of Investments [Line Items]
Total fixed maturities, available-for-sale	4,136	5,316
Gross Unrealized Gains	834	1,530
Gross Unrealized Losses	156	145
Total fixed maturities, available-for-sale	4,814	6,701
OTTI in AOCI	0	0
Fixed maturities, available-for-sale: | All other U.S. public corporate securities
Summary of Investments [Line Items]
Total fixed maturities, available-for-sale	1,802,350	2,138,358
Gross Unrealized Gains	67,908	81,905
Gross Unrealized Losses	28,846	61,142
Total fixed maturities, available-for-sale	1,841,412	2,159,121
OTTI in AOCI	(215)	(217)
Fixed maturities, available-for-sale: | All other U.S. private corporate securities
Summary of Investments [Line Items]
Total fixed maturities, available-for-sale	714,776	1,085,345
Gross Unrealized Gains	14,555	26,299
Gross Unrealized Losses	7,702	13,963
Total fixed maturities, available-for-sale	721,629	1,097,681
OTTI in AOCI	(236)	0
Fixed maturities, available-for-sale: | All other foreign public corporate securities
Summary of Investments [Line Items]
Total fixed maturities, available-for-sale	216,428	270,063
Gross Unrealized Gains	7,371	8,230
Gross Unrealized Losses	4,127	6,508
Total fixed maturities, available-for-sale	219,672	271,785
OTTI in AOCI	0	0
Fixed maturities, available-for-sale: | All other foreign private corporate securities
Summary of Investments [Line Items]
Total fixed maturities, available-for-sale	577,761	784,283
Gross Unrealized Gains	4,866	9,933
Gross Unrealized Losses	33,455	42,528
Total fixed maturities, available-for-sale	549,172	751,688
OTTI in AOCI	0	0
Fixed maturities, available-for-sale: | Asset-backed securities
Summary of Investments [Line Items]
Total fixed maturities, available-for-sale	184,414	431,578
Gross Unrealized Gains	5,164	6,203
Gross Unrealized Losses	562	2,650
Total fixed maturities, available-for-sale	189,016	435,131
OTTI in AOCI	(2,534)	(3,056)
Fixed maturities, available-for-sale: | Commercial mortgage-backed securities
Summary of Investments [Line Items]
Total fixed maturities, available-for-sale	382,717	396,160
Gross Unrealized Gains	5,783	10,614
Gross Unrealized Losses	5,829	2,429
Total fixed maturities, available-for-sale	382,671	404,345
OTTI in AOCI	0	0
Fixed maturities, available-for-sale: | Residential mortgage-backed securities
Summary of Investments [Line Items]
Total fixed maturities, available-for-sale	80,141	114,943
Gross Unrealized Gains	3,355	7,876
Gross Unrealized Losses	308	65
Total fixed maturities, available-for-sale	83,188	122,754
OTTI in AOCI	(274)	(690)
Equity securities, available-for-sale
Summary of Investments [Line Items]
Equity securities, available-for-sale, cost	16,390	54,609
Gross Unrealized Gains	518	423
Gross Unrealized Losses	152	3,059
Equity securities, available-for-sale, at fair value	16,756	51,973
Equity securities, available-for-sale | Mutual funds
Summary of Investments [Line Items]
Equity securities, available-for-sale, cost	16,324	54,543
Gross Unrealized Gains	441	256
Gross Unrealized Losses	124	3,030
Equity securities, available-for-sale, at fair value	16,641	51,769
Equity securities, available-for-sale | Public Utility, Equities
Summary of Investments [Line Items]
Equity securities, available-for-sale, cost	66	66
Gross Unrealized Gains	2	2
Gross Unrealized Losses	28	29
Equity securities, available-for-sale, at fair value	40	39
Equity securities, available-for-sale | Industrial, miscellaneous & other
Summary of Investments [Line Items]
Equity securities, available-for-sale, cost	0	0
Gross Unrealized Gains	75	165
Gross Unrealized Losses	0	0
Equity securities, available-for-sale, at fair value	$ 75	$ 165