Fair Value of Financial Instruments - Financial Instruments Measured and Carried at Fair Value (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	$ 5,839,560,000	$ 7,515,318,000
Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	754,355,000	787,441,000
Other, including U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	458,175,000	833,542,000
Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	1,676,422,000	2,003,969,000
Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	47,915,000	84,904,000
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	1,585,108,000	2,079,510,000
Other mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	1,317,585,000	1,725,952,000
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	6,206,797,000	7,765,437,000
Separate Account (variable annuity) assets	2,308,134,000	2,001,128,000
Investment contract and policy reserves, embedded derivatives	1,145,000	248,000
Other policyholder funds, embedded derivatives	89,098,000	78,700,000
Carrying Amount | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	5,839,560,000	7,515,318,000
Carrying Amount | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	237,819,000	122,222,000
Carrying Amount | Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	754,355,000	787,441,000
Carrying Amount | Other, including U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	458,175,000	833,542,000
Carrying Amount | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	1,676,422,000	2,003,969,000
Carrying Amount | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	47,915,000	84,904,000
Carrying Amount | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	1,585,108,000	2,079,510,000
Carrying Amount | Other mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	1,317,585,000	1,725,952,000
Carrying Amount | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	105,483,000	111,750,000
Carrying Amount | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	23,935,000	16,147,000
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	6,206,797,000	7,765,437,000
Separate Account (variable annuity) assets	2,308,134,000	2,001,128,000
Investment contract and policy reserves, embedded derivatives	1,145,000	248,000
Other policyholder funds, embedded derivatives	89,098,000	78,700,000
Fair Value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	5,839,560,000	7,515,318,000
Fair Value | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	237,819,000	122,222,000
Fair Value | Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	754,355,000	787,441,000
Fair Value | Other, including U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	458,175,000	833,542,000
Fair Value | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	1,676,422,000	2,003,969,000
Fair Value | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	47,915,000	84,904,000
Fair Value | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	1,585,108,000	2,079,510,000
Fair Value | Other mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	1,317,585,000	1,725,952,000
Fair Value | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	105,483,000	111,750,000
Fair Value | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	23,935,000	16,147,000
Fair Value | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	327,980,000	207,198,000
Separate Account (variable annuity) assets	2,308,134,000	2,001,128,000
Investment contract and policy reserves, embedded derivatives	0	0
Other policyholder funds, embedded derivatives	0	0
Fair Value | Level 1 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	32,285,000	25,572,000
Fair Value | Level 1 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	237,819,000	117,296,000
Fair Value | Level 1 | Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	0	0
Fair Value | Level 1 | Other, including U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	17,779,000	13,291,000
Fair Value | Level 1 | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	0	0
Fair Value | Level 1 | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	0	0
Fair Value | Level 1 | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	14,506,000	12,281,000
Fair Value | Level 1 | Other mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	0	0
Fair Value | Level 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	57,876,000	64,330,000
Fair Value | Level 1 | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	0	0
Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	5,587,618,000	7,309,750,000
Separate Account (variable annuity) assets	0	0
Investment contract and policy reserves, embedded derivatives	1,145,000	248,000
Other policyholder funds, embedded derivatives	0	0
Fair Value | Level 2 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	5,516,192,000	7,241,262,000
Fair Value | Level 2 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	0	4,926,000
Fair Value | Level 2 | Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	749,551,000	784,224,000
Fair Value | Level 2 | Other, including U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	440,396,000	820,251,000
Fair Value | Level 2 | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	1,628,717,000	1,956,438,000
Fair Value | Level 2 | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	47,915,000	84,904,000
Fair Value | Level 2 | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	1,473,929,000	1,986,487,000
Fair Value | Level 2 | Other mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	1,175,684,000	1,608,958,000
Fair Value | Level 2 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	47,491,000	47,415,000
Fair Value | Level 2 | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	23,935,000	16,147,000
Fair Value | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	291,199,000	248,489,000
Separate Account (variable annuity) assets	0	0
Investment contract and policy reserves, embedded derivatives	0	0
Other policyholder funds, embedded derivatives	89,098,000	78,700,000
Fair Value | Level 3 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	291,083,000	248,484,000
Fair Value | Level 3 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	0	0
Fair Value | Level 3 | Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	4,804,000	3,217,000
Fair Value | Level 3 | Other, including U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	0	0
Fair Value | Level 3 | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	47,705,000	47,531,000
Fair Value | Level 3 | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	0	0
Fair Value | Level 3 | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	96,673,000	80,742,000
Fair Value | Level 3 | Other mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	141,901,000	116,994,000
Fair Value | Level 3 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	116,000	5,000
Fair Value | Level 3 | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of issued securities	$ 0	$ 0