Fair Value of Financial Instruments - Financial Instruments Measured and Carried at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Carrying Amount
U.S. Government and federally sponsored agency obligations:
Investments	$ 7,950,184	$ 7,663,469
Investment contract and life policy reserves, embedded derivatives	594	158
Other policyholder funds, embedded derivatives	80,733	59,393
Carrying Amount | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	7,724,075	7,456,708
Carrying Amount | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	62,593	44,918
Carrying Amount | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	696,725	442,419
Carrying Amount | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	735,408	467,143
Carrying Amount | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	1,893,253	1,769,397
Carrying Amount | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	102,738	98,669
Carrying Amount | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	2,578,954	2,810,221
Carrying Amount | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	1,716,997	1,868,859
Carrying Amount | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	135,466	141,649
Carrying Amount | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	28,050	20,194
Fair Value
U.S. Government and federally sponsored agency obligations:
Investments	7,950,184	7,663,469
Investment contract and life policy reserves, embedded derivatives	594	158
Other policyholder funds, embedded derivatives	80,733	59,393
Fair Value | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	7,724,075	7,456,708
Fair Value | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	62,593	44,918
Fair Value | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	696,725	442,419
Fair Value | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	735,408	467,143
Fair Value | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	1,893,253	1,769,397
Fair Value | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	102,738	98,669
Fair Value | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	2,578,954	2,810,221
Fair Value | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	1,716,997	1,868,859
Fair Value | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	135,466	141,649
Fair Value | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	28,050	20,194
Fair Value | Level 1
U.S. Government and federally sponsored agency obligations:
Investments	172,539	169,962
Investment contract and life policy reserves, embedded derivatives	0	0
Other policyholder funds, embedded derivatives	0	0
Fair Value | Level 1 | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	27,738	27,163
Fair Value | Level 1 | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	62,593	44,167
Fair Value | Level 1 | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	0	0
Fair Value | Level 1 | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	13,393	13,631
Fair Value | Level 1 | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	0	0
Fair Value | Level 1 | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	0	0
Fair Value | Level 1 | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	14,345	13,532
Fair Value | Level 1 | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	0	0
Fair Value | Level 1 | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	82,208	98,632
Fair Value | Level 1 | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	0	0
Fair Value | Level 2
U.S. Government and federally sponsored agency obligations:
Investments	7,547,388	7,281,403
Investment contract and life policy reserves, embedded derivatives	594	158
Other policyholder funds, embedded derivatives	0	0
Fair Value | Level 2 | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	7,466,086	7,218,198
Fair Value | Level 2 | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	0	0
Fair Value | Level 2 | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	693,375	439,004
Fair Value | Level 2 | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	722,015	453,512
Fair Value | Level 2 | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	1,843,925	1,722,900
Fair Value | Level 2 | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	102,738	98,669
Fair Value | Level 2 | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	2,491,630	2,736,498
Fair Value | Level 2 | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	1,612,403	1,767,615
Fair Value | Level 2 | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	53,252	43,011
Fair Value | Level 2 | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	28,050	20,194
Fair Value | Level 3
U.S. Government and federally sponsored agency obligations:
Investments	230,257	212,104
Investment contract and life policy reserves, embedded derivatives	0	0
Other policyholder funds, embedded derivatives	80,733	59,393
Fair Value | Level 3 | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	230,251	211,347
Fair Value | Level 3 | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	0	751
Fair Value | Level 3 | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	3,350	3,415
Fair Value | Level 3 | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	0	0
Fair Value | Level 3 | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	49,328	46,497
Fair Value | Level 3 | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	0	0
Fair Value | Level 3 | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	72,979	60,191
Fair Value | Level 3 | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	104,594	101,244
Fair Value | Level 3 | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	6	6
Fair Value | Level 3 | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	$ 0	$ 0