Investments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	$ 2,616,274	$ 2,520,643
Securities, Gross Unrealized Gains	259,299	229,528
Securities, Gross Unrealized Losses	22,139	31,340
Available for sale, fair value	2,853,434	2,718,831
U.S. Treasury securities and obligations of U.S. Government [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	121,606	120,593
Securities, Gross Unrealized Gains	14,308	13,856
Securities, Gross Unrealized Losses	22	12
Available for sale, fair value	135,892	134,437
Federal agencies [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	22,063	22,401
Securities, Gross Unrealized Gains	4,061	3,480
Securities, Gross Unrealized Losses	1
Available for sale, fair value	26,123	25,881
Federal agency issued residential mortgage-backed securities [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	97,158	109,738
Securities, Gross Unrealized Gains	9,281	9,901
Securities, Gross Unrealized Losses		2
Available for sale, fair value	106,439	119,637
Corporate obligations [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	1,911,439	1,802,977
Securities, Gross Unrealized Gains	196,514	176,444
Securities, Gross Unrealized Losses	4,963	8,535
Available for sale, fair value	2,102,990	1,970,886
Corporate private-labeled residential mortgage-backed securities [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	157,621	167,666
Securities, Gross Unrealized Gains	2,711	1,856
Securities, Gross Unrealized Losses	8,694	12,620
Available for sale, fair value	151,638	156,902
Municipal securities [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	148,664	150,267
Securities, Gross Unrealized Gains	25,510	18,316
Securities, Gross Unrealized Losses	26	61
Available for sale, fair value	174,148	168,522
Others [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	106,488	100,315
Securities, Gross Unrealized Gains	4,771	3,576
Securities, Gross Unrealized Losses	8,081	9,235
Available for sale, fair value	103,178	94,656
Fixed maturity securities [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	2,580,775	2,485,692
Securities, Gross Unrealized Gains	257,484	227,655
Securities, Gross Unrealized Losses	22,009	31,205
Available for sale, fair value	2,816,250	2,682,142
Equity securities [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	35,499	34,951
Securities, Gross Unrealized Gains	1,815	1,873
Securities, Gross Unrealized Losses	130	135
Available for sale, fair value	37,184	36,689
Redeemable preferred stocks [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	15,736	11,735
Securities, Gross Unrealized Gains	328	226
Securities, Gross Unrealized Losses	222	740
Available for sale, fair value	15,842	11,221
US Treasury Securities [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	240,827	252,732
Securities, Gross Unrealized Gains	27,650	27,237
Securities, Gross Unrealized Losses	23	14
Available for sale, fair value	268,454	279,955
Industrial [Member] | Corporate obligations [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	498,231	444,030
Securities, Gross Unrealized Gains	48,437	43,710
Securities, Gross Unrealized Losses	1,727	860
Available for sale, fair value	544,941	486,880
Energy [Member] | Corporate obligations [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	173,030	152,580
Securities, Gross Unrealized Gains	20,692	19,131
Securities, Gross Unrealized Losses	46
Available for sale, fair value	193,676	171,711
Communications and Technology [Member] | Corporate obligations [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	199,416	184,983
Securities, Gross Unrealized Gains	20,097	16,566
Securities, Gross Unrealized Losses	28	156
Available for sale, fair value	219,485	201,393
Financial [Member] | Corporate obligations [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	298,693	308,813
Securities, Gross Unrealized Gains	20,753	15,155
Securities, Gross Unrealized Losses	2,624	5,890
Available for sale, fair value	316,822	318,078
Consumer [Member] | Corporate obligations [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	488,138	452,962
Securities, Gross Unrealized Gains	47,467	43,788
Securities, Gross Unrealized Losses	53	263
Available for sale, fair value	535,552	496,487
Public Utilities [Member] | Corporate obligations [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Securities, Amortized Cost	253,931	259,609
Securities, Gross Unrealized Gains	39,068	38,094
Securities, Gross Unrealized Losses	485	1,366
Available for sale, fair value	292,514	296,337
Fixed maturity securities [Member] | U.S. Treasury securities and obligations of U.S. Government [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Available for sale, fair value	135,892	134,437
Fixed maturity securities [Member] | Federal agencies [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Available for sale, fair value	26,123	25,881
Fixed maturity securities [Member] | Federal agency issued residential mortgage-backed securities [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Available for sale, fair value	106,439	119,637
Fixed maturity securities [Member] | Corporate obligations [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Available for sale, fair value	2,102,990	1,970,886
Fixed maturity securities [Member] | Corporate private-labeled residential mortgage-backed securities [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Available for sale, fair value	151,638	156,902
Fixed maturity securities [Member] | Municipal securities [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Available for sale, fair value	174,148	168,522
Fixed maturity securities [Member] | Others [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Available for sale, fair value	103,178	94,656
Fixed maturity securities [Member] | Redeemable preferred stocks [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Available for sale, fair value	15,842	11,221
Fixed maturity securities [Member] | US Treasury Securities [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Available for sale, fair value	268,454	279,955
Fixed maturity securities [Member] | Industrial [Member] | Corporate obligations [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Available for sale, fair value	544,941	486,880
Fixed maturity securities [Member] | Energy [Member] | Corporate obligations [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Available for sale, fair value	193,676	171,711
Fixed maturity securities [Member] | Communications and Technology [Member] | Corporate obligations [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Available for sale, fair value	219,485	201,393
Fixed maturity securities [Member] | Financial [Member] | Corporate obligations [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Available for sale, fair value	316,822	318,078
Fixed maturity securities [Member] | Consumer [Member] | Corporate obligations [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Available for sale, fair value	535,552	496,487
Fixed maturity securities [Member] | Public Utilities [Member] | Corporate obligations [Member]
Schedule of Amortized Cost and Fair Value of Securities [Abstract]
Available for sale, fair value	$ 292,514	$ 296,337