Fair Value of Financial Instruments - Carrying Values and Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands		Sep. 30, 2018		Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities, at fair value		$ 2,010,900	[1]	$ 1,927,842	[2]
Fixed-maturity security held to maturity, fair value		874,328		779,472
Available-for-sale equity securities, at fair value				41,107
Equity securities		39,317
Trading securities		15,955		6,228
Separate accounts assets		2,419,997		2,572,872
Surplus note		891,139		736,381
Separate accounts liabilities		2,419,997		2,572,872
Fixed-maturity security (held-to-maturity)		891,860		737,150
Reported Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities, at fair value		2,010,900		1,927,842
Available-for-sale equity securities, at fair value				41,107
Equity securities		39,317
Trading securities		15,955		6,228
Separate accounts assets		2,419,997		2,572,872
Separate accounts liabilities		2,419,997		2,572,872
Reported Value Measurement [Member] | Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Policy loans	[3]	30,902		32,816
Surplus note	[3],[4]	891,139		736,381
Fixed-maturity security (held-to-maturity)	[3]	891,860		737,150
Reported Value Measurement [Member] | Level 3 [Member] | Deposits [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Deposit asset underlying 10% coinsurance agreement	[3]	226,025		217,336
Reported Value Measurement [Member] | Senior Notes [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Notes payable	[4],[5]	373,567		373,288
Estimate of Fair Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale debt securities, at fair value		2,010,900		1,927,842
Available-for-sale equity securities, at fair value				41,107
Equity securities		39,317
Trading securities		15,955		6,228
Separate accounts assets		2,419,997		2,572,872
Separate accounts liabilities		2,419,997		2,572,872
Estimate of Fair Value Measurement [Member] | Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed-maturity security held to maturity, fair value	[3]	874,328		779,472
Policy loans	[3]	30,902		32,816
Surplus note	[3],[4]	874,328		778,050
Estimate of Fair Value Measurement [Member] | Level 3 [Member] | Deposits [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Deposit asset underlying 10% coinsurance agreement	[3]	226,025		217,336
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Notes payable	[4],[5]	$ 386,505		$ 400,628

[1] Includes $0.2 million of other-than-temporary impairment ("OTTI") losses related to corporates and mortgage- and asset-backed securities recognized in accumulated other comprehensive income (loss).

[2] Includes $0.2 million of OTTI losses related to corporates and mortgage- and asset-backed securities recognized in accumulated other comprehensive income (loss).

[3] Classified as a level 3 fair value measurement.

[4] Carrying value amounts shown are net of issuance costs

[5] Classified as a level 2 fair value measurement.