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, 2016		Jun. 30, 2015		Jun. 30, 2016		Jun. 30, 2015
Unrealized investment gains (losses) on available for sale assets:
Realized (gains) losses	$ (4,005)	[1]	$ (2,613)	[2]	$ (4,298)	[1]	$ (2,732)	[2]
Amortization of (discount) premium	(201,642)	[1]	(194,823)	[2]	(398,695)	[1]	(386,419)	[2]
Total before tax	(199,344)	[1]	(196,723)	[2]	(394,792)	[1]	(391,200)	[2]
Tax	60,050	[1]	64,196	[2]	121,924	[1]	127,895	[2]
Net income	(138,429)	[1]	(127,110)	[2]	(262,462)	[1]	(248,758)	[2]
Pension adjustments:
Total reclassifications (after tax)	(1,714)		374		(1,145)		1,575
Unrealized Investment Gains (Losses) on Available for Sale Assets [Member]
Pension adjustments:
Total reclassifications (after tax)	(3,372)		(2,001)		(4,462)		(3,165)
Unrealized Investment Gains (Losses) on Available for Sale Assets [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Unrealized investment gains (losses) on available for sale assets:
Realized (gains) losses	(3,983)		(1,479)		(4,296)		(1,598)
Amortization of (discount) premium	(1,204)		(1,599)		(2,568)		(3,271)
Total before tax	(5,187)		(3,078)		(6,864)		(4,869)
Tax	1,815		1,077		2,402		1,704
Available for Sale Assets [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Unrealized investment gains (losses) on available for sale assets:
Net income	(3,372)		(2,001)		(4,462)		(3,165)
Amortization of Prior Service Cost [Member]
Pension adjustments:
Pension adjustments reclassified out of accumulated other comprehensive income	120		82		240		163
Amortization of Actuarial Gain (Loss) [Member]
Pension adjustments:
Pension adjustments reclassified out of accumulated other comprehensive income	2,431		3,571		4,863		7,128
Pension Adjustments [Member]
Pension adjustments:
Pension adjustments reclassified out of accumulated other comprehensive income	2,551		3,653		5,103		7,291
Tax	(893)		(1,278)		(1,786)		(2,551)
Total reclassifications (after tax)	$ 1,658		$ 2,375		$ 3,317		$ 4,740

[1] Due to the adoption of ASU 2016-09, certain balances related to excess tax benefits from stock compensation were adjusted prospectively as described in Note 2—New Accounting Standards.

[2] Certain prior year balances were adjusted to give effect to discontinued operations as described in Note 5—Discontinued Operations.