Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
U.S. Government and federally sponsored agency obligations:
Carrying Amount	$ 7,551,390	$ 7,379,290
Fair Value	7,551,390	7,379,290
Investment contract and life policy reserves, embedded derivatives, Carrying Amount	23	14
Investment contract and life policy reserves, embedded derivatives, Fair Value Disclosure	23	14
Other policyholder funds, embedded derivatives, Carrying Amount	42,085	39,021
Other policyholder funds, embedded derivatives, Fair Value Disclosure	42,085	39,021
Recurring [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	7,216,502	7,091,340
Fair Value	7,216,502	7,091,340
Short-term investments [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	216,204	174,152
Fair Value	216,204	174,152
Mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	495,200	504,414
Fair Value	495,200	504,414
Other, including U.S. Treasury Securities [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	540,120	546,111
Fair Value	540,120	546,111
Municipal bonds [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	1,727,328	1,708,943
Fair Value	1,727,328	1,708,943
Foreign government bonds [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	74,087	73,617
Fair Value	74,087	73,617
Corporate bonds [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	2,833,931	2,779,415
Fair Value	2,833,931	2,779,415
Other mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	1,545,836	1,478,840
Fair Value	1,545,836	1,478,840
Equity securities [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	104,044	99,797
Fair Value	104,044	99,797
Other investments [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	14,640	14,001
Fair Value	14,640	14,001
Level 1 [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	323,215	280,305
Investment contract and life policy reserves, embedded derivatives, Fair Value Disclosure	0	0
Other policyholder funds, embedded derivatives, Fair Value Disclosure	0	0
Level 1 [Member] | Recurring [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	22,574	24,453
Level 1 [Member] | Short-term investments [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	210,320	169,764
Level 1 [Member] | Mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	0	0
Level 1 [Member] | Other, including U.S. Treasury Securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	13,863	14,258
Level 1 [Member] | Municipal bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	0	0
Level 1 [Member] | Foreign government bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	0	0
Level 1 [Member] | Corporate bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	8,711	10,195
Level 1 [Member] | Other mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	0	0
Level 1 [Member] | Equity securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	90,321	86,088
Level 1 [Member] | Other investments [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	0	0
Level 2 [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	7,027,784	6,925,559
Investment contract and life policy reserves, embedded derivatives, Fair Value Disclosure	23	14
Other policyholder funds, embedded derivatives, Fair Value Disclosure	0	0
Level 2 [Member] | Recurring [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	6,993,543	6,893,467
Level 2 [Member] | Short-term investments [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	5,884	4,388
Level 2 [Member] | Mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	493,001	504,414
Level 2 [Member] | Other, including U.S. Treasury Securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	526,257	531,853
Level 2 [Member] | Municipal bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	1,680,835	1,678,564
Level 2 [Member] | Foreign government bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	74,087	73,617
Level 2 [Member] | Corporate bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	2,755,149	2,701,645
Level 2 [Member] | Other mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	1,464,214	1,403,374
Level 2 [Member] | Equity securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	13,717	13,703
Level 2 [Member] | Other investments [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	14,640	14,001
Level 3 [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	200,391	173,426
Investment contract and life policy reserves, embedded derivatives, Fair Value Disclosure	0	0
Other policyholder funds, embedded derivatives, Fair Value Disclosure	42,085	39,021
Level 3 [Member] | Recurring [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	200,385	173,420
Level 3 [Member] | Short-term investments [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	0	0
Level 3 [Member] | Mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	2,199	0
Level 3 [Member] | Other, including U.S. Treasury Securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	0	0
Level 3 [Member] | Municipal bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	46,493	30,379
Level 3 [Member] | Foreign government bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	0	0
Level 3 [Member] | Corporate bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	70,071	67,575
Level 3 [Member] | Other mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	81,622	75,466
Level 3 [Member] | Equity securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	6	6
Level 3 [Member] | Other investments [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	$ 0	$ 0