Fair Value of Financial Instruments - Financial Instruments Measured and Carried at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Carrying Amount
U.S. Government and federally sponsored agency obligations:
Investments	$ 7,765,437	$ 7,950,184
Investment contract and life policy reserves, embedded derivatives	248	594
Other policyholder funds, embedded derivatives	78,700	80,733
Carrying Amount | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	7,515,318	7,724,075
Carrying Amount | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	122,222	62,593
Carrying Amount | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	787,441	696,725
Carrying Amount | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	833,542	735,408
Carrying Amount | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	2,003,969	1,893,253
Carrying Amount | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	84,904	102,738
Carrying Amount | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	2,079,510	2,578,954
Carrying Amount | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	1,725,952	1,716,997
Carrying Amount | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	111,750	135,466
Carrying Amount | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	16,147	28,050
Fair Value
U.S. Government and federally sponsored agency obligations:
Investments	7,765,437	7,950,184
Investment contract and life policy reserves, embedded derivatives	248	594
Other policyholder funds, embedded derivatives	78,700	80,733
Fair Value | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	7,515,318	7,724,075
Fair Value | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	122,222	62,593
Fair Value | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	787,441	696,725
Fair Value | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	833,542	735,408
Fair Value | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	2,003,969	1,893,253
Fair Value | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	84,904	102,738
Fair Value | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	2,079,510	2,578,954
Fair Value | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	1,725,952	1,716,997
Fair Value | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	111,750	135,466
Fair Value | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	16,147	28,050
Fair Value | Level 1
U.S. Government and federally sponsored agency obligations:
Investments	207,198	172,539
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	25,572	27,738
Fair Value | Level 1 | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	117,296	62,593
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	13,291	13,393
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	12,281	14,345
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	64,330	82,208
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	7,309,750	7,547,388
Investment contract and life policy reserves, embedded derivatives	248	594
Other policyholder funds, embedded derivatives	0	0
Fair Value | Level 2 | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	7,241,262	7,466,086
Fair Value | Level 2 | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	4,926	0
Fair Value | Level 2 | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	784,224	693,375
Fair Value | Level 2 | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	820,251	722,015
Fair Value | Level 2 | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	1,956,438	1,843,925
Fair Value | Level 2 | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	84,904	102,738
Fair Value | Level 2 | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	1,986,487	2,491,630
Fair Value | Level 2 | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	1,608,958	1,612,403
Fair Value | Level 2 | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	47,415	53,252
Fair Value | Level 2 | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	16,147	28,050
Fair Value | Level 3
U.S. Government and federally sponsored agency obligations:
Investments	248,489	230,257
Investment contract and life policy reserves, embedded derivatives	0	0
Other policyholder funds, embedded derivatives	78,700	80,733
Fair Value | Level 3 | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	248,484	230,251
Fair Value | Level 3 | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	0	0
Fair Value | Level 3 | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	3,217	3,350
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	47,531	49,328
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	80,742	72,979
Fair Value | Level 3 | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	116,994	104,594
Fair Value | Level 3 | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	5	6
Fair Value | Level 3 | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	$ 0	$ 0