Fair Values Valuation of Financial Instruments by Hierarchy Levels (Details) - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		$ 7,484,622	$ 7,033,045
Other investments		21,198	5,757
Assets held in separate accounts		625,177	561,281
Other Investments		48,833	33,765
Reported Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments		29,135	29,020
Future policy benefits - index product embedded derivatives		4,237,938	4,217,904
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments		19,698	4,745
Cash and short-term investments		23,375	34,748
Reinsurance recoverable		824	157
Assets held in separate accounts		625,177	561,281
Assets, Fair Value Disclosure		8,254,316	7,723,532
Other liabilities		193	780
Total liabilities		59,568	40,808
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments		0	0
Cash and short-term investments		23,375	34,748
Reinsurance recoverable		0	0
Assets held in separate accounts		625,177	561,281
Assets, Fair Value Disclosure		672,683	609,207
Future policy benefits - index product embedded derivatives		0	0
Other liabilities		0	0
Total liabilities		0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments		19,698	4,745
Cash and short-term investments		0	0
Reinsurance recoverable		824	157
Assets held in separate accounts		0	0
Assets, Fair Value Disclosure		7,530,627	7,013,911
Future policy benefits - index product embedded derivatives		0	0
Other liabilities		193	780
Total liabilities		193	780
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments		0	0
Cash and short-term investments		0	0
Reinsurance recoverable		0	0
Assets held in separate accounts		0	0
Assets, Fair Value Disclosure		51,006	100,414
Other liabilities		0	0
Total liabilities		59,375	40,028
Fair Value, Measurements, Recurring | Fair Value, 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		51,006	100,414
Debt Securities | 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		7,484,622	7,033,045
Debt Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		7,382	7,917
Debt Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		7,433,282	6,931,576
Debt Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		43,958	93,552
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		3,585,750	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		3,585,750	3,279,885
Corporate | Fair Value, Measurements, Recurring | Fair Value, Inputs, 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 | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		3,566,052	3,257,874
Corporate | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		19,698	22,011
Corporate | Fair Value, Measurements, Recurring | Fair Value, 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		19,698	22,011
Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		629,379	606,860
Residential mortgage-backed securities | 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		629,379	606,860
Residential mortgage-backed securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities available-for-sale		0	0
Residential mortgage-backed securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities available-for-sale		627,255	606,860
Residential mortgage-backed securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities available-for-sale		2,124	0
Residential mortgage-backed securities | Fair Value, Measurements, Recurring | Fair Value, 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		2,124
Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		1,003,755	878,566
Commercial mortgage-backed securities | 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		1,003,755	878,566
Commercial mortgage-backed securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities available-for-sale		0	0
Commercial mortgage-backed securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities available-for-sale		995,421	810,626
Commercial mortgage-backed securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities available-for-sale		8,334	67,940
Commercial mortgage-backed securities | Fair Value, Measurements, Recurring | Fair Value, 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		8,334	67,940
Other asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		691,447	707,570
Other asset backed securities | 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		691,447	707,570
Other asset backed securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities available-for-sale		0	0
Other asset backed securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities available-for-sale		677,645	703,969
Other asset backed securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities available-for-sale		13,802	3,601
Other asset backed securities | Fair Value, Measurements, Recurring | Fair Value, 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		13,802	3,601
United States Government and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		18,585	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		18,585	20,535
United States Government and agencies | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		7,382	7,917
United States Government and agencies | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		11,203	12,618
United States Government and agencies | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		0	0
State, municipal and other government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		1,555,706	1,539,629
State, municipal and other government | 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		1,555,706	1,539,629
State, municipal and other government | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		0	0
State, municipal and other government | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		1,555,706	1,539,629
State, municipal and other government | Fair Value, Measurements, Recurring | Fair Value, 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 stock | 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		83,871	84,295
Non-redeemable preferred stock | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		0	0
Non-redeemable preferred stock | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		76,823	77,433
Non-redeemable preferred stock | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		7,048	6,862
Common Stock | 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]	16,749	5,261
Common Stock | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		16,749	5,261
Common Stock | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		0	0
Common Stock | Fair Value, Measurements, Recurring | Fair Value, 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 Stock
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		4,700	6,800
Private Equity Funds, US | Common Stock | Fair Value Measured at Net Asset Value Per Share
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments Net Asset Value		5,400	3,300
Index Product Embedded Derivatives | Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Future policy benefits - index product embedded derivatives		59,375	40,028
Index Product Embedded Derivatives | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Future policy benefits - index product embedded derivatives		$ 59,375	$ 40,028

[1] A private equity fund with a fair value estimate of $5.4 million at June 30, 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 $4.7 million at June 30, 2019 and $6.8 million at December 31, 2018. The investment is not currently eligible for redemption.