Supplemental Information about Changes to Accumulated Other Comprehensive Income - Summary of Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Unrealized investment gains (losses) on available for sale assets:
Realized (gains) losses	$ (11,813)	$ 705	$ (13,764)	$ 6,453
Amortization of (discount) premium	(218,568)	(212,776)	(436,652)	(421,058)
Total before tax	(226,864)	(201,926)	(439,706)	(393,667)
Tax	42,471	61,563	81,602	116,126
Net income	(184,425)	(140,273)	(358,025)	(273,814)
Pension adjustments:
Total reclassifications (after tax)	(3,779)	2,424	(1,344)	3,488
Unrealized Investment Gains (Losses) on Available for Sale Assets [Member]
Pension adjustments:
Total reclassifications (after tax)	(6,763)	403	(7,313)	(554)
Pension Adjustments [Member]
Pension adjustments:
Total reclassifications (after tax)	2,984	2,021	5,969	4,042
Reclassification out of Accumulated Other Comprehensive Income [Member]
Pension adjustments:
Total reclassifications (after tax)	(3,779)	2,424	(1,344)	3,488
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Unrealized Investment Gains (Losses) on Available for Sale Assets [Member]
Unrealized investment gains (losses) on available for sale assets:
Realized (gains) losses	(9,431)	681	(10,817)	(354)
Amortization of (discount) premium	870	(61)	1,560	(498)
Total before tax	(8,561)	620	(9,257)	(852)
Tax	1,798	(217)	1,944	298
Net income	(6,763)	403	(7,313)	(554)
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Amortization of Prior Service Cost [Member]
Pension adjustments:
Pension adjustments	119	119	238	238
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Amortization of Actuarial Gain (Loss) [Member]
Pension adjustments:
Pension adjustments	3,659	2,990	7,318	5,980
Reclassification out of Accumulated Other Comprehensive Income [Member] \| Pension Adjustments [Member]
Pension adjustments:
Pension adjustments reclassified out of accumulated other comprehensive income	3,778	3,109	7,556	6,218
Tax	(794)	(1,088)	(1,587)	(2,176)
Total reclassifications (after tax)	$ 2,984	$ 2,021	$ 5,969	$ 4,042

Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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Amount of income (expense) related to nonoperating activities, classified as other.

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Amount before tax of reclassification adjustments of other comprehensive income (loss).

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