Fair Value of Financial Instruments - Financial Instruments Measured and Carried at Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 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)	$ 1,356	$ 4,739	$ (110)	$ 5,935
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	2,107	3,397	3,439	6,786
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	271,651	246,505	271,651	246,505	$ 243,868	$ 230,257	$ 248,757	$ 212,104
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value	77,788	67,995	77,788	67,995	78,486	80,733	64,261	59,393
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	48,031	11,483	73,631	61,654
Transfers into Level 3	0	0	0	0
Net realized investment gains (losses) included in net income related to financial assets	(246)	(1,714)	(243)	(1,714)
Net realized gains (losses) included in net income related to financial liabilities	(1,291)	1,238	(3,513)	3,546
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	15,509	11,410	15,509	18,271
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
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	1,514	901	2,871	1,730
Paydowns, maturities and distributions	5,849	5,350	16,372	13,203
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)	397	1,287	840	3,158
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	49,921	49,123	49,921	49,123	49,748	49,328	53,462	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	5,557	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	224	69	247	189
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)	(700)	359	(1,587)	455
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	92,663	77,052	92,663	77,052	78,780	72,979	82,495	60,191
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	29,709	2,001	40,487	31,919
Net realized investment gains (losses) included in net income related to financial assets	(246)	0	(246)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	11,279	5,853	11,279	11,963
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	3,601	1,950	7,691	3,550
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	7,804,718		7,804,718			7,950,184
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	486		486			594
Other policyholder funds, embedded derivatives	77,788		77,788			80,733
Carrying Amount | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	7,475,565		7,475,565			7,724,075
Carrying Amount | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	176,497		176,497			62,593
Carrying Amount | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	707,324		707,324			696,725
Carrying Amount | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	782,223		782,223			735,408
Carrying Amount | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	1,827,511		1,827,511			1,893,253
Carrying Amount | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	95,505		95,505			102,738
Carrying Amount | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	2,397,724		2,397,724			2,578,954
Carrying Amount | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	1,665,278		1,665,278			1,716,997
Carrying Amount | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	130,780		130,780			135,466
Carrying Amount | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	21,876		21,876			28,050
Fair Value
U.S. Government and federally sponsored agency obligations:
Investments	7,804,718		7,804,718			7,950,184
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	486		486			594
Other policyholder funds, embedded derivatives	77,788		77,788			80,733
Fair Value | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	7,475,565		7,475,565			7,724,075
Fair Value | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	176,497		176,497			62,593
Fair Value | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	707,324		707,324			696,725
Fair Value | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	782,223		782,223			735,408
Fair Value | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	1,827,511		1,827,511			1,893,253
Fair Value | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	95,505		95,505			102,738
Fair Value | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	2,397,724		2,397,724			2,578,954
Fair Value | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	1,665,278		1,665,278			1,716,997
Fair Value | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	130,780		130,780			135,466
Fair Value | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	21,876		21,876			28,050
Fair Value | Level 1
U.S. Government and federally sponsored agency obligations:
Investments	283,331		283,331			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	26,598		26,598			27,738
Fair Value | Level 1 | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	176,497		176,497			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	13,201		13,201			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	13,397		13,397			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	80,236		80,236			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	7,249,736		7,249,736			7,547,388
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	486		486			594
Other policyholder funds, embedded derivatives	0		0			0
Fair Value | Level 2 | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	7,177,322		7,177,322			7,466,086
Fair Value | Level 2 | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	0		0			0
Fair Value | Level 2 | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	703,543		703,543			693,375
Fair Value | Level 2 | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	769,022		769,022			722,015
Fair Value | Level 2 | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	1,777,590		1,777,590			1,843,925
Fair Value | Level 2 | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	95,505		95,505			102,738
Fair Value | Level 2 | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	2,291,664		2,291,664			2,491,630
Fair Value | Level 2 | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	1,539,998		1,539,998			1,612,403
Fair Value | Level 2 | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	50,538		50,538			53,252
Fair Value | Level 2 | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	21,876		21,876			28,050
Fair Value | Level 3
U.S. Government and federally sponsored agency obligations:
Investments	271,651		271,651			230,257
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	0		0			0
Other policyholder funds, embedded derivatives	77,788		77,788			80,733
Fair Value | Level 3 | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	271,645		271,645			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	3,781		3,781			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	49,921		49,921			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	92,663		92,663			72,979
Fair Value | Level 3 | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	125,280		125,280			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