Fair Value of Financial Instruments - Financial Instruments Measured and Carried at Fair Value (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Financial Assets
Fixed maturity securities	$ 5,843,183,000	$ 5,791,676,000
Equity securities	87,033,000	101,864,000
Mortgage-backed securities
Financial Assets
Fixed maturity securities	751,915,000	724,319,000
Other, including U.S. Treasury securities
Financial Assets
Fixed maturity securities	468,673,000	458,868,000
Municipal bonds
Financial Assets
Fixed maturity securities	1,769,931,000	1,686,203,000
Foreign government bonds
Financial Assets
Fixed maturity securities	43,852,000	45,370,000
Corporate bonds
Financial Assets
Fixed maturity securities	1,685,773,000	1,581,424,000
Other mortgage-backed securities
Financial Assets
Fixed maturity securities	1,123,039,000	1,295,492,000
Level 3
Financial Assets
Equity securities	83,000
Financial Liabilities
Other policyholder funds, embedded derivatives	87,506,000
Level 3 | Mortgage-backed securities
Financial Assets
Fixed maturity securities	23,332,000
Level 3 | Municipal bonds
Financial Assets
Fixed maturity securities	104,892,000
Level 3 | Corporate bonds
Financial Assets
Fixed maturity securities	111,693,000
Level 3 | Other mortgage-backed securities
Financial Assets
Fixed maturity securities	117,195,000
Carrying Amount
Financial Assets
Equity securities	87,033,000	101,864,000
Short-term investments	100,208,000	172,667,000
Other investments	18,147,000	25,997,000
Totals	6,048,571,000	6,092,204,000
Separate Account (variable annuity) assets	1,954,201,000	2,490,469,000
Financial Liabilities
Investment contract and policy reserves, embedded derivatives	339,000	1,314,000
Other policyholder funds, embedded derivatives	87,506,000	93,733,000
Carrying Amount | Recurring
Financial Assets
Fixed maturity securities	5,843,183,000	5,791,676,000
Carrying Amount | Mortgage-backed securities
Financial Assets
Fixed maturity securities	751,915,000	724,319,000
Carrying Amount | Other, including U.S. Treasury securities
Financial Assets
Fixed maturity securities	468,673,000	458,868,000
Carrying Amount | Municipal bonds
Financial Assets
Fixed maturity securities	1,769,931,000	1,686,203,000
Carrying Amount | Foreign government bonds
Financial Assets
Fixed maturity securities	43,851,000	45,370,000
Carrying Amount | Corporate bonds
Financial Assets
Fixed maturity securities	1,685,773,000	1,581,424,000
Carrying Amount | Other mortgage-backed securities
Financial Assets
Fixed maturity securities	1,123,040,000	1,295,492,000
Fair Value
Financial Assets
Equity securities	87,033,000	101,864,000
Short-term investments	100,208,000	172,667,000
Other investments	18,147,000	25,997,000
Totals	6,048,571,000	6,092,204,000
Separate Account (variable annuity) assets	1,954,201,000	2,490,469,000
Financial Liabilities
Investment contract and policy reserves, embedded derivatives	339,000	1,314,000
Other policyholder funds, embedded derivatives	87,506,000	93,733,000
Fair Value | Recurring
Financial Assets
Fixed maturity securities	5,843,183,000	5,791,676,000
Fair Value | Mortgage-backed securities
Financial Assets
Fixed maturity securities	751,915,000	724,319,000
Fair Value | Other, including U.S. Treasury securities
Financial Assets
Fixed maturity securities	468,673,000	458,868,000
Fair Value | Municipal bonds
Financial Assets
Fixed maturity securities	1,769,931,000	1,686,203,000
Fair Value | Foreign government bonds
Financial Assets
Fixed maturity securities	43,851,000	45,370,000
Fair Value | Corporate bonds
Financial Assets
Fixed maturity securities	1,685,773,000	1,581,424,000
Fair Value | Other mortgage-backed securities
Financial Assets
Fixed maturity securities	1,123,040,000	1,295,492,000
Fair Value | Level 1
Financial Assets
Equity securities	42,106,000	49,834,000
Short-term investments	97,179,000	172,667,000
Other investments	0	0
Totals	169,989,000	254,670,000
Separate Account (variable annuity) assets	1,954,201,000	2,490,469,000
Financial Liabilities
Investment contract and policy reserves, embedded derivatives	0	0
Other policyholder funds, embedded derivatives	0	0
Fair Value | Level 1 | Recurring
Financial Assets
Fixed maturity securities	30,704,000	32,169,000
Fair Value | Level 1 | Mortgage-backed securities
Financial Assets
Fixed maturity securities	0	0
Fair Value | Level 1 | Other, including U.S. Treasury securities
Financial Assets
Fixed maturity securities	18,557,000	17,699,000
Fair Value | Level 1 | Municipal bonds
Financial Assets
Fixed maturity securities	0	0
Fair Value | Level 1 | Foreign government bonds
Financial Assets
Fixed maturity securities	0	0
Fair Value | Level 1 | Corporate bonds
Financial Assets
Fixed maturity securities	12,147,000	14,470,000
Fair Value | Level 1 | Other mortgage-backed securities
Financial Assets
Fixed maturity securities	0	0
Fair Value | Level 2
Financial Assets
Equity securities	44,844,000	51,923,000
Short-term investments	3,029,000	0
Other investments	18,147,000	25,997,000
Totals	5,521,387,000	5,542,356,000
Separate Account (variable annuity) assets	0	0
Financial Liabilities
Investment contract and policy reserves, embedded derivatives	339,000	1,314,000
Other policyholder funds, embedded derivatives	0	0
Fair Value | Level 2 | Recurring
Financial Assets
Fixed maturity securities	5,455,367,000	5,464,436,000
Fair Value | Level 2 | Mortgage-backed securities
Financial Assets
Fixed maturity securities	728,583,000	711,004,000
Fair Value | Level 2 | Other, including U.S. Treasury securities
Financial Assets
Fixed maturity securities	450,116,000	441,169,000
Fair Value | Level 2 | Municipal bonds
Financial Assets
Fixed maturity securities	1,665,039,000	1,641,912,000
Fair Value | Level 2 | Foreign government bonds
Financial Assets
Fixed maturity securities	43,851,000	45,370,000
Fair Value | Level 2 | Corporate bonds
Financial Assets
Fixed maturity securities	1,561,933,000	1,463,002,000
Fair Value | Level 2 | Other mortgage-backed securities
Financial Assets
Fixed maturity securities	1,005,845,000	1,161,979,000
Fair Value | Level 3
Financial Assets
Equity securities	83,000	107,000
Short-term investments	0	0
Other investments	0	0
Totals	357,195,000	295,178,000
Separate Account (variable annuity) assets	0	0
Financial Liabilities
Investment contract and policy reserves, embedded derivatives	0	0
Other policyholder funds, embedded derivatives	87,506,000	93,733,000
Fair Value | Level 3 | Recurring
Financial Assets
Fixed maturity securities	357,112,000	295,071,000
Fair Value | Level 3 | Mortgage-backed securities
Financial Assets
Fixed maturity securities	23,332,000	13,315,000
Fair Value | Level 3 | Other, including U.S. Treasury securities
Financial Assets
Fixed maturity securities	0	0
Fair Value | Level 3 | Municipal bonds
Financial Assets
Fixed maturity securities	104,892,000	44,291,000
Fair Value | Level 3 | Foreign government bonds
Financial Assets
Fixed maturity securities	0	0
Fair Value | Level 3 | Corporate bonds
Financial Assets
Fixed maturity securities	111,693,000	103,952,000
Fair Value | Level 3 | Other mortgage-backed securities
Financial Assets
Fixed maturity securities	$ 117,195,000	$ 133,513,000