Fair Value of Financial Instruments - Financial Instruments Measured and Carried at Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 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)	$ (1,466)	$ 1,196
Net unrealized investment gains (losses) included in other comprehensive income		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0
Purchases		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances	0	0
Issuances	1,332	3,389
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	243,868	248,757	$ 230,257	$ 212,104
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	78,486	64,261	80,733	59,393
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	25,600	50,171
Transfers into Level 3	0	0
Net realized investment gains (losses) included in net income related to financial assets	3	0
Net realized gains (losses) included in net income related to financial liabilities	(2,222)	2,308
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	6,861
Transfers out of Level 3	0	0
Financial Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	3	0
Sales		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0
Settlements		0
FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPaydownsAndMaturities	1,357	829
Paydowns, maturities and distributions	10,523	7,853
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
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	0	0	0	751
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Net realized investment gains (losses) included in net income related to financial assets	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	751
Financial Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0
Paydowns, maturities and distributions	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)	443	1,871
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	49,748	53,462	49,328	46,497
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	5,214
Net realized investment gains (losses) included in net income related to financial assets	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0
Financial Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0
Paydowns, maturities and distributions	23	120
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)	(887)	96
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	78,780	82,495	72,979	60,191
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	10,778	29,918
Net realized investment gains (losses) included in net income related to financial assets	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	6,110
Financial Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0
Paydowns, maturities and distributions	4,090	1,600
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
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	6	6	6	$ 6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Net realized investment gains (losses) included in net income related to financial assets	3	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0
Financial Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	3	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0
Paydowns, maturities and distributions	0	$ 0
Carrying Amount
U.S. Government and federally sponsored agency obligations:
Investments	7,813,029		7,950,184
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	518		594
Other policyholder funds, embedded derivatives	78,486		80,733
Carrying Amount | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	7,634,682		7,724,075
Carrying Amount | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	22,194		62,593
Carrying Amount | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	710,958		696,725
Carrying Amount | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	761,967		735,408
Carrying Amount | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	1,861,304		1,893,253
Carrying Amount | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	99,041		102,738
Carrying Amount | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	2,481,270		2,578,954
Carrying Amount | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	1,720,142		1,716,997
Carrying Amount | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	131,023		135,466
Carrying Amount | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	25,130		28,050
Fair Value
U.S. Government and federally sponsored agency obligations:
Investments	7,813,029		7,950,184
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	518		594
Other policyholder funds, embedded derivatives	78,486		80,733
Fair Value | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	7,634,682		7,724,075
Fair Value | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	22,194		62,593
Fair Value | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	710,958		696,725
Fair Value | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	761,967		735,408
Fair Value | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	1,861,304		1,893,253
Fair Value | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	99,041		102,738
Fair Value | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	2,481,270		2,578,954
Fair Value | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	1,720,142		1,716,997
Fair Value | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	131,023		135,466
Fair Value | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	25,130		28,050
Fair Value | Level 1
U.S. Government and federally sponsored agency obligations:
Investments	129,046		172,539
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	0		0
Other policyholder funds, embedded derivatives	0		0
Fair Value | Level 1 | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	26,982		27,738
Fair Value | Level 1 | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	22,194		62,593
Fair Value | Level 1 | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	0		0
Fair Value | Level 1 | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	13,249		13,393
Fair Value | Level 1 | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	0		0
Fair Value | Level 1 | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	0		0
Fair Value | Level 1 | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	13,733		14,345
Fair Value | Level 1 | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	0		0
Fair Value | Level 1 | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	79,870		82,208
Fair Value | Level 1 | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	0		0
Fair Value | Level 2
U.S. Government and federally sponsored agency obligations:
Investments	7,440,115		7,547,388
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	518		594
Other policyholder funds, embedded derivatives	0		0
Fair Value | Level 2 | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	7,363,838		7,466,086
Fair Value | Level 2 | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	0		0
Fair Value | Level 2 | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	707,842		693,375
Fair Value | Level 2 | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	748,718		722,015
Fair Value | Level 2 | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	1,811,556		1,843,925
Fair Value | Level 2 | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	99,041		102,738
Fair Value | Level 2 | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	2,388,757		2,491,630
Fair Value | Level 2 | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	1,607,924		1,612,403
Fair Value | Level 2 | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	51,147		53,252
Fair Value | Level 2 | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	25,130		28,050
Fair Value | Level 3
U.S. Government and federally sponsored agency obligations:
Investments	243,868		230,257
Financial Liabilities
Investment contract and life policy reserves, embedded derivatives	0		0
Other policyholder funds, embedded derivatives	78,486		80,733
Fair Value | Level 3 | Recurring
U.S. Government and federally sponsored agency obligations:
Investments	243,862		230,251
Fair Value | Level 3 | Short-term investments
U.S. Government and federally sponsored agency obligations:
Investments	0		0
Fair Value | Level 3 | Mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	3,116		3,350
Fair Value | Level 3 | Other, including U.S. Treasury securities
U.S. Government and federally sponsored agency obligations:
Investments	0		0
Fair Value | Level 3 | Municipal bonds
U.S. Government and federally sponsored agency obligations:
Investments	49,748		49,328
Fair Value | Level 3 | Foreign government bonds
U.S. Government and federally sponsored agency obligations:
Investments	0		0
Fair Value | Level 3 | Corporate bonds
U.S. Government and federally sponsored agency obligations:
Investments	78,780		72,979
Fair Value | Level 3 | Other mortgage-backed securities
U.S. Government and federally sponsored agency obligations:
Investments	112,218		104,594
Fair Value | Level 3 | Equity securities
U.S. Government and federally sponsored agency obligations:
Investments	6		6
Fair Value | Level 3 | Other investments
U.S. Government and federally sponsored agency obligations:
Investments	$ 0		$ 0