Investments - Summary of Fixed Maturities Available for Sale by Component (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 14,651,551	$ 14,188,050
Fair Value	16,318,286	$ 15,245,861
Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	14,651,551
Gross Unrealized Gains	1,769,632
Gross Unrealized Losses	(102,897)
Fair Value	$ 16,318,286
Percentage of Total Fixed Maturities At Fair Value	100.00%
Fixed maturities available for sale [Member] \| U.S. Government direct, guaranteed, and government-sponsored enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 387,331
Gross Unrealized Gains	8,552
Gross Unrealized Losses	(1,696)
Fair Value	$ 394,187
Percentage of Total Fixed Maturities At Fair Value	2.00%
Fixed maturities available for sale [Member] \| States, municipalities, and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,182,708
Gross Unrealized Gains	128,175
Gross Unrealized Losses	(185)
Fair Value	$ 1,310,698
Percentage of Total Fixed Maturities At Fair Value	8.00%
Fixed maturities available for sale [Member] \| Foreign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 20,720
Gross Unrealized Gains	1,592
Gross Unrealized Losses	0
Fair Value	$ 22,312
Percentage of Total Fixed Maturities At Fair Value	0.00%
Fixed maturities available for sale [Member] \| Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 12,498,820
Gross Unrealized Gains	1,550,098
Gross Unrealized Losses	(84,629)
Fair Value	$ 13,964,289
Percentage of Total Fixed Maturities At Fair Value	87.00%
Fixed maturities available for sale [Member] \| Corporates [Member] \| Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 3,152,297
Gross Unrealized Gains	413,004
Gross Unrealized Losses	(25,309)
Fair Value	$ 3,539,992
Percentage of Total Fixed Maturities At Fair Value	22.00%
Fixed maturities available for sale [Member] \| Corporates [Member] \| Utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,909,247
Gross Unrealized Gains	314,825
Gross Unrealized Losses	(3,351)
Fair Value	$ 2,220,721
Percentage of Total Fixed Maturities At Fair Value	14.00%
Fixed maturities available for sale [Member] \| Corporates [Member] \| Energy [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,589,769
Gross Unrealized Gains	170,566
Gross Unrealized Losses	(35,952)
Fair Value	$ 1,724,383
Percentage of Total Fixed Maturities At Fair Value	11.00%
Fixed maturities available for sale [Member] \| Corporates [Member] \| Other corporate sectors [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 5,847,507
Gross Unrealized Gains	651,703
Gross Unrealized Losses	(20,017)
Fair Value	$ 6,479,193
Percentage of Total Fixed Maturities At Fair Value	40.00%
Fixed maturities available for sale [Member] \| Collateralized debt obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 59,871
Gross Unrealized Gains	16,677
Gross Unrealized Losses	(10,303)
Fair Value	$ 66,245
Percentage of Total Fixed Maturities At Fair Value	0.00%
Fixed maturities available for sale [Member] \| Other asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 126,019
Gross Unrealized Gains	3,607
Gross Unrealized Losses	(15)
Fair Value	$ 129,611
Percentage of Total Fixed Maturities At Fair Value	1.00%
Fixed maturities available for sale [Member] \| Redeemable preferred stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 376,082
Gross Unrealized Gains	60,931
Gross Unrealized Losses	(6,069)
Fair Value	$ 430,944
Percentage of Total Fixed Maturities At Fair Value	2.00%
Fixed maturities available for sale [Member] \| Redeemable preferred stocks [Member] \| Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 347,505
Gross Unrealized Gains	59,206
Gross Unrealized Losses	(5,849)
Fair Value	$ 400,862
Percentage of Total Fixed Maturities At Fair Value	2.00%
Fixed maturities available for sale [Member] \| Redeemable preferred stocks [Member] \| Utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 28,577
Gross Unrealized Gains	1,725
Gross Unrealized Losses	(220)
Fair Value	$ 30,082
Percentage of Total Fixed Maturities At Fair Value	0.00%

Percentage of fixed maturities by component at fair value.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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