Fair Value of Financial Instruments - Carrying Values and Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value		$ 1,968,949	$ 1,837,332
Fixed-maturity securities held to maturity, fair value		779,472	513,015
Trading securities		6,228	7,383
Surplus note		736,381	502,491
Fixed-maturity securities (held-to-maturity)		737,150	503,230
Fair Value, Recurring Measurements [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading securities		6,228	7,383
Level 2 [Member] | Fair Value, Recurring Measurements [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading securities		4,800	7,332
Separate Accounts Assets, Fair Value Disclosure [Member] | Fair Value, Recurring Measurements [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Separate accounts assets		2,572,872	2,287,953
Separate Accounts Assets, Fair Value Disclosure [Member] | Level 2 [Member] | Fair Value, Recurring Measurements [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Separate accounts assets		2,572,872	2,287,953
Separate Accounts Liabilities [Member] | Fair Value, Recurring Measurements [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Separate accounts liabilities		2,572,872	2,287,953
Separate Accounts Liabilities [Member] | Level 2 [Member] | Fair Value, Recurring Measurements [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Separate accounts liabilities		2,572,872	2,287,953
Reported Value Measurement [Member] | Fair Value, Recurring Measurements [Member] | Fixed-maturity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value		1,927,842	1,792,438
Reported Value Measurement [Member] | Fair Value, Recurring Measurements [Member] | Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value		41,107	44,894
Reported Value Measurement [Member] | Level 3 [Member] | Fair value, Nonrecurring Measurements [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Policy loans		32,816	30,916
Surplus note	[1]	736,381	502,491
Fixed-maturity securities (held-to-maturity)		737,150	503,230
Reported Value Measurement [Member] | Level 3 [Member] | Fair value, Nonrecurring Measurements [Member] | Deposits [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Deposit asset underlying 10% coinsurance agreement		217,336	202,435
Reported Value Measurement [Member] | Level 2 [Member] | Fair Value, Recurring Measurements [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading securities		6,228	7,383
Reported Value Measurement [Member] | Separate Accounts Assets, Fair Value Disclosure [Member] | Level 2 [Member] | Fair Value, Recurring Measurements [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Separate accounts assets		2,572,872	2,287,953
Reported Value Measurement [Member] | Senior Notes [Member] | Level 2 [Member] | Fair value, Nonrecurring Measurements [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Notes payable	[1]	373,288	372,919
Reported Value Measurement [Member] | Separate Accounts Liabilities [Member] | Level 2 [Member] | Fair Value, Recurring Measurements [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Separate accounts liabilities		2,572,872	2,287,953
Estimate of Fair Value Measurement [Member] | Fair Value, Recurring Measurements [Member] | Fixed-maturity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value		1,927,842	1,792,438
Estimate of Fair Value Measurement [Member] | Fair Value, Recurring Measurements [Member] | Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, fair value		41,107	44,894
Estimate of Fair Value Measurement [Member] | Level 3 [Member] | Fair value, Nonrecurring Measurements [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed-maturity securities held to maturity, fair value		779,472	513,015
Policy loans		32,816	30,916
Surplus note	[1]	778,050	512,669
Estimate of Fair Value Measurement [Member] | Level 3 [Member] | Fair value, Nonrecurring Measurements [Member] | Deposits [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Deposit asset underlying 10% coinsurance agreement		217,336	202,435
Estimate of Fair Value Measurement [Member] | Level 2 [Member] | Fair Value, Recurring Measurements [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading securities		6,228	7,383
Estimate of Fair Value Measurement [Member] | Separate Accounts Assets, Fair Value Disclosure [Member] | Level 2 [Member] | Fair Value, Recurring Measurements [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Separate accounts assets		2,572,872	2,287,953
Estimate of Fair Value Measurement [Member] | Senior Notes [Member] | Level 2 [Member] | Fair value, Nonrecurring Measurements [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Notes payable	[1]	400,628	401,340
Estimate of Fair Value Measurement [Member] | Separate Accounts Liabilities [Member] | Level 2 [Member] | Fair Value, Recurring Measurements [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Separate accounts liabilities		$ 2,572,872	$ 2,287,953

[1] Carrying value amounts shown are net of issuance costs.