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
Mar. 31, 2018	Mar. 31, 2017
Unrealized investment gains (losses) on available for sale assets:
Realized (gains) losses	$ (1,951)	$ 5,748
Amortization of (discount) premium	(218,084)	(208,282)
Total before tax	(212,842)	(191,741)
Tax	39,131	54,563
Net income	(173,600)	(133,541)
Pension adjustments:
Total reclassifications (after tax)	2,435	1,064
Unrealized Investment Gains (Losses) on Available for Sale Assets [Member]
Pension adjustments:
Total reclassifications (after tax)	(550)	(957)
Amortization of Prior Service Cost [Member]
Pension adjustments:
Pension adjustments reclassified out of accumulated other comprehensive income	119	119
Amortization of Actuarial Gain (Loss) [Member]
Pension adjustments:
Pension adjustments reclassified out of accumulated other comprehensive income	3,659	2,990
Pension Adjustments [Member]
Pension adjustments:
Pension adjustments reclassified out of accumulated other comprehensive income	3,778	3,109
Tax	(793)	(1,088)
Total reclassifications (after tax)	2,985	2,021
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	(1,386)	(1,035)
Amortization of (discount) premium	690	(437)
Total before tax	(696)	(1,472)
Tax	146	515
Net income	$ (550)	$ (957)

This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting.

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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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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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

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Amount after tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

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