Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost	$ 7,194,397	$ 7,152,127
Amortized Cost or Cost	140,200	134,013
Fair Value	7,630,634	7,456,708
Fair Value	159,275	141,649
Federal National Mortgage Association Certificates and Obligations (FNMA)
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	332,057	272,668
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	373,676	378,683
Government National Mortgage Association Certificates and Obligations (GNMA)
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	109,873	115,627
Total Fixed Maturity Securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost	7,194,397	7,152,127
Unrealized Investment Gains	460,619	374,991
Unrealized Investment Losses	24,382	70,410
Fair Value	7,630,634	7,456,708
OTTI in AOCI	1,335	1,618
Mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost	613,761	587,355
Unrealized Investment Gains	34,712	34,256
Unrealized Investment Losses	3,071	6,720
Fair Value	645,402	614,891
OTTI in AOCI	0	0
Other, including U.S. Treasury securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost	653,237	458,745
Unrealized Investment Gains	24,705	18,518
Unrealized Investment Losses	6,922	10,120
Fair Value	671,020	467,143
OTTI in AOCI	0	0
Municipal bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost	1,669,273	1,648,252
Unrealized Investment Gains	177,359	143,733
Unrealized Investment Losses	4,428	22,588
Fair Value	1,842,204	1,769,397
OTTI in AOCI	0	0
Foreign government bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost	93,761	93,864
Unrealized Investment Gains	6,416	5,102
Unrealized Investment Losses	0	297
Fair Value	100,177	98,669
OTTI in AOCI	0	0
Corporate bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost	2,635,313	2,672,818
Unrealized Investment Gains	190,548	152,229
Unrealized Investment Losses	4,158	14,826
Fair Value	2,821,703	2,810,221
OTTI in AOCI	0	0
Other mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost	1,529,052	1,691,093
Unrealized Investment Gains	26,879	21,153
Unrealized Investment Losses	5,803	15,859
Fair Value	1,550,128	1,696,387
OTTI in AOCI	1,335	1,618
Equity securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost	140,200	134,013
Unrealized Investment Gains	20,483	13,210
Unrealized Investment Losses	1,408	5,574
Fair Value	159,275	141,649
OTTI in AOCI	$ 0	$ 0

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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