Fair Value of Financial Instruments - Financial Instruments Measured and Carried at Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	$ (6,527)	$ (187)	$ (6,637)	$ 5,748
Net unrealized investment gains (losses) included in other comprehensive income	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0	0	0
Purchases	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances	0	0	0	0
Issuances	3,940	3,752	7,379	10,538
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	261,484	265,081	261,484	265,081	$ 271,651	$ 230,257	$ 246,505	$ 212,104
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value	82,265	72,986	82,265	72,986	77,788	80,733	67,995	59,393
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	17,030	35,462	90,661	97,116
Transfers into Level 3	0	0	0	0
Net realized investment gains (losses) included in net income related to financial assets	0	(161)	(243)	(1,875)
Net realized gains (losses) included in net income related to financial liabilities	2,205	2,587	(1,308)	6,133
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	970	880	16,479	19,151
Transfers out of Level 3	0	0	0	0
Financial Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	187	1,999	190	1,999
Sales	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0	0	0
Settlements	0	0	0	0
FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPaydownsAndMaturities	(1,668)	1,348	(4,539)	3,078
Paydowns, maturities and distributions	19,513	13,659	35,885	26,862
Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	0	0	0	0	0	0	0	751
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0	0	0
Net realized investment gains (losses) included in net income related to financial assets	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0	0	751
Financial Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0	0	0
Paydowns, maturities and distributions	0	0	0	0
Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	(471)	382	369	3,540
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	49,329	49,212	49,329	49,212	49,921	49,328	49,123	46,497
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0	0	5,214
Net realized investment gains (losses) included in net income related to financial assets	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0	0	5,557
Financial Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0	0	0
Paydowns, maturities and distributions	121	293	368	482
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	128	(192)	(1,459)	263
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	88,865	94,245	88,865	94,245	92,663	72,979	77,052	60,191
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	23,501	40,487	55,420
Net realized investment gains (losses) included in net income related to financial assets	0	(1)	(246)	(1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	(1)	11,279	11,962
Financial Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	1,999	0	1,999
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0	0	0
Paydowns, maturities and distributions	3,926	4,117	11,617	7,667
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	6	6	6	6	$ 6	6	$ 6	$ 6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0	0	0
Net realized investment gains (losses) included in net income related to financial assets	0	0	3	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0	0	0
Financial Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	3	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0	0	0
Paydowns, maturities and distributions	0	$ 0	0	$ 0
Carrying Amount
U.S. Government and federally sponsored agency obligations:
Investments	7,759,668		7,759,668			7,950,184
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	722		722			594
Other policyholder funds, embedded derivatives	82,265		82,265			80,733
Carrying Amount | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	7,529,139		7,529,139			7,724,075
Carrying Amount | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	74,647		74,647			62,593
Carrying Amount | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	750,836		750,836			696,725
Carrying Amount | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	782,600		782,600			735,408
Carrying Amount | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	1,883,674		1,883,674			1,893,253
Carrying Amount | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	95,003		95,003			102,738
Carrying Amount | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	2,335,496		2,335,496			2,578,954
Carrying Amount | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	1,681,530		1,681,530			1,716,997
Carrying Amount | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	133,184		133,184			135,466
Carrying Amount | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	22,698		22,698			28,050
Fair Value
U.S. Government and federally sponsored agency obligations:
Investments	7,759,668		7,759,668			7,950,184
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	722		722			594
Other policyholder funds, embedded derivatives	82,265		82,265			80,733
Fair Value | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	7,529,139		7,529,139			7,724,075
Fair Value | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	74,647		74,647			62,593
Fair Value | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	750,836		750,836			696,725
Fair Value | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	782,600		782,600			735,408
Fair Value | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	1,883,674		1,883,674			1,893,253
Fair Value | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	95,003		95,003			102,738
Fair Value | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	2,335,496		2,335,496			2,578,954
Fair Value | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	1,681,530		1,681,530			1,716,997
Fair Value | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	133,184		133,184			135,466
Fair Value | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	22,698		22,698			28,050
Fair Value | Level 1
U.S. Government and federally sponsored agency obligations:
Investments	179,383		179,383			172,539
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	0		0			0
Other policyholder funds, embedded derivatives	0		0			0
Fair Value | Level 1 | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	26,289		26,289			27,738
Fair Value | Level 1 | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	70,705		70,705			62,593
Fair Value | Level 1 | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	0		0			0
Fair Value | Level 1 | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	13,101		13,101			13,393
Fair Value | Level 1 | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	0		0			0
Fair Value | Level 1 | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	0		0			0
Fair Value | Level 1 | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	13,188		13,188			14,345
Fair Value | Level 1 | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	0		0			0
Fair Value | Level 1 | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	82,389		82,389			82,208
Fair Value | Level 1 | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	0		0			0
Fair Value | Level 2
U.S. Government and federally sponsored agency obligations:
Investments	7,318,801		7,318,801			7,547,388
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	722		722			594
Other policyholder funds, embedded derivatives	0		0			0
Fair Value | Level 2 | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	7,241,372		7,241,372			7,466,086
Fair Value | Level 2 | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	3,942		3,942			0
Fair Value | Level 2 | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	747,469		747,469			693,375
Fair Value | Level 2 | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	769,499		769,499			722,015
Fair Value | Level 2 | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	1,834,345		1,834,345			1,843,925
Fair Value | Level 2 | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	95,003		95,003			102,738
Fair Value | Level 2 | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	2,233,443		2,233,443			2,491,630
Fair Value | Level 2 | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	1,561,613		1,561,613			1,612,403
Fair Value | Level 2 | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	50,789		50,789			53,252
Fair Value | Level 2 | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	22,698		22,698			28,050
Fair Value | Level 3
U.S. Government and federally sponsored agency obligations:
Investments	261,484		261,484			230,257
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	0		0			0
Other policyholder funds, embedded derivatives	82,265		82,265			80,733
Fair Value | Level 3 | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	261,478		261,478			230,251
Fair Value | Level 3 | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	0		0			0
Fair Value | Level 3 | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	3,367		3,367			3,350
Fair Value | Level 3 | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	0		0			0
Fair Value | Level 3 | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	49,329		49,329			49,328
Fair Value | Level 3 | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	0		0			0
Fair Value | Level 3 | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	88,865		88,865			72,979
Fair Value | Level 3 | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	119,917		119,917			104,594
Fair Value | Level 3 | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	6		6			6
Fair Value | Level 3 | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	$ 0		$ 0			$ 0