Fair Value Valuation of our Financial Instruments Measured on a Recurring Basis by Hierarchy Levels (Details) - USD ($) $ in Thousands		Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments		$ 34,181	$ 5,757
Assets held in separate accounts		645,881	561,281
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		7,702,628	7,033,045
Other investments		31,469	4,745
Cash, cash equivalents and short-term investments		29,142	34,748
Reinsurance recoverable		2,327	157
Assets held in separate accounts		645,881	561,281
Assets, Fair Value Disclosure		8,503,274	7,723,532
Future policy benefits - indexed product embedded derivatives		76,346	40,028
Other liabilities		254	780
Liabilities, Fair Value Disclosure		76,600	40,808
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		4,821	7,917
Other investments		0	0
Cash, cash equivalents and short-term investments		29,142	34,748
Reinsurance recoverable		0	0
Assets held in separate accounts		645,881	561,281
Assets, Fair Value Disclosure		696,871	609,207
Future policy benefits - indexed product embedded derivatives		0	0
Other liabilities		0	0
Liabilities, Fair Value Disclosure		0	0
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		7,668,684	6,931,576
Other investments		31,469	4,745
Cash, cash equivalents and short-term investments		0	0
Reinsurance recoverable		2,327	157
Assets held in separate accounts		0	0
Assets, Fair Value Disclosure		7,770,353	7,013,911
Future policy benefits - indexed product embedded derivatives		0	0
Other liabilities		254	780
Liabilities, Fair Value Disclosure		254	780
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		29,123	93,552
Other investments		0	0
Cash, cash equivalents and short-term investments		0	0
Reinsurance recoverable		0	0
Assets held in separate accounts		0	0
Assets, Fair Value Disclosure		36,050	100,414
Other liabilities		0	0
Liabilities, Fair Value Disclosure		76,346	40,028
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		36,050	100,414
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		3,778,950	3,279,885
Corporate | Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		3,778,950	3,279,885
Corporate | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		0	0
Corporate | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		3,772,362	3,257,874
Corporate | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		6,588	22,011
Corporate | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		6,588	22,011
Residential mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		672,388	606,860
Residential mortgage-backed | Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure		672,388	606,860
Residential mortgage-backed | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure		0	0
Residential mortgage-backed | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure		672,388	606,860
Residential mortgage-backed | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure		0	0
Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		1,045,473	878,566
Commercial mortgage-backed | Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure		1,045,473	878,566
Commercial mortgage-backed | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure		0	0
Commercial mortgage-backed | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure		1,032,693	810,626
Commercial mortgage-backed | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure		12,780	67,940
Commercial mortgage-backed | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		12,780	67,940
Other asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		714,521	707,570
Other asset-backed | Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure		714,521	707,570
Other asset-backed | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure		0	0
Other asset-backed | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure		704,766	703,969
Other asset-backed | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure		9,755	3,601
Other asset-backed | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		9,755	3,601
United States Government and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		14,123	20,535
United States Government and agencies | Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		14,123	20,535
United States Government and agencies | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		4,821	7,917
United States Government and agencies | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		9,302	12,618
United States Government and agencies | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		0	0
States and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		1,477,173	1,539,629
States and political subdivisions | Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		1,477,173	1,539,629
States and political subdivisions | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		0	0
States and political subdivisions | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		1,477,173	1,539,629
States and political subdivisions | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		0	0
Non-redeemable preferred stocks | Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		74,800	84,295
Non-redeemable preferred stocks | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		0	0
Non-redeemable preferred stocks | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		67,873	77,433
Non-redeemable preferred stocks | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		6,927	6,862
Non-redeemable preferred stocks | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		6,927	6,862
Common stocks | Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	[1]	17,027	5,261
Common stocks | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		17,027	5,261
Common stocks | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		0	0
Common stocks | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		0	0
Private Equity Funds, US | Common stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments		1,700	6,800
Private Equity Funds, US | Common stocks | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Alternative Investment		$ 8,400	$ 3,300

[1] A private equity fund with a fair value estimate of $8.4 million at December 31, 2019 and $3.3 million at December 31, 2018 using net asset value per share as a practical expedient, has not been classified in the fair value hierarchy above in accordance with fair value reporting guidance. This fund invests in senior secured middle market loans and had unfunded commitments totaling $1.7 million at December 31, 2019 and $6.8 million at December 31, 2018. The investment is not currently eligible for redemption.