Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
U.S. Government and federally sponsored agency obligations:
Carrying Amount	$ 7,663,469	$ 7,379,290
Fair Value	7,663,469	7,379,290
Investment contract and life policy reserves, embedded derivatives, Carrying Amount	158	14
Investment contract and life policy reserves, embedded derivatives, Fair Value Disclosure	158	14
Other policyholder funds, embedded derivatives, Carrying Amount	59,393	39,021
Other policyholder funds, embedded derivatives, Fair Value Disclosure	59,393	39,021
Recurring [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	7,456,708	7,091,340
Fair Value	7,456,708	7,091,340
Short-term investments [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	44,918	174,152
Fair Value	44,918	174,152
Mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	442,419	504,414
Fair Value	442,419	504,414
Other, including U.S. Treasury Securities [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	467,143	546,111
Fair Value	467,143	546,111
Municipal bonds [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	1,769,397	1,708,943
Fair Value	1,769,397	1,708,943
Foreign government bonds [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	98,669	73,617
Fair Value	98,669	73,617
Corporate bonds [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	2,810,221	2,779,415
Fair Value	2,810,221	2,779,415
Other mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	1,868,859	1,478,840
Fair Value	1,868,859	1,478,840
Equity securities [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	141,649	99,797
Fair Value	141,649	99,797
Other Investment [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	20,194	14,001
Fair Value	20,194	14,001
Level 1 [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	169,962	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	27,163	24,453
Level 1 [Member] | Short-term investments [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	44,167	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	13,631	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	13,532	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	98,632	86,088
Level 1 [Member] | Other Investment [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	7,281,403	6,925,559
Investment contract and life policy reserves, embedded derivatives, Fair Value Disclosure	158	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	7,218,198	6,893,467
Level 2 [Member] | Short-term investments [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	0	4,388
Level 2 [Member] | Mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	439,004	504,414
Level 2 [Member] | Other, including U.S. Treasury Securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	453,512	531,853
Level 2 [Member] | Municipal bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	1,722,900	1,678,564
Level 2 [Member] | Foreign government bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	98,669	73,617
Level 2 [Member] | Corporate bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	2,736,498	2,701,645
Level 2 [Member] | Other mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	1,767,615	1,403,374
Level 2 [Member] | Equity securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	43,011	13,703
Level 2 [Member] | Other Investment [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	20,194	14,001
Level 3 [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	212,104	173,426
Investment contract and life policy reserves, embedded derivatives, Fair Value Disclosure	0	0
Other policyholder funds, embedded derivatives, Fair Value Disclosure	59,393	39,021
Level 3 [Member] | Recurring [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	211,347	173,420
Level 3 [Member] | Short-term investments [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	751	0
Level 3 [Member] | Mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	3,415	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	46,497	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	60,191	67,575
Level 3 [Member] | Other mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	101,244	75,466
Level 3 [Member] | Equity securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	6	6
Level 3 [Member] | Other Investment [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	$ 0	$ 0