Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Components of Other Comprehensive Income (Loss) [Abstract]
Net unrealized investment (losses) gains arising during the year										$ (33,718)	$ 91,782	$ (126,175)
Income tax benefit (expense)										11,738	(32,017)	43,732
Valuation allowance	[1]									62,383	31,374	(41,148)
Net of taxes										40,403	91,139	(123,591)
Net changes in benefit plan assets and obligations										(12,818)	(52,112)	68,038
Income tax benefit (expense)										4,487	18,239	(23,813)
Valuation allowance	[1]									(7,383)	(18,239)	23,813
Net of taxes										(15,714)	(52,112)	68,038
Net changes in unrealized foreign currency translation adjustment										(5,699)	(4,067)	(21,563)
Income tax benefit										2,000	1,425	7,553
Valuation allowance	[1]									(529)	0	0
Net of taxes										(4,228)	(2,642)	(14,010)
Total other comprehensive (loss) income										(52,235)	35,603	(79,700)
Total income tax benefit, net of valuation allowance										72,696	782	10,137
Other comprehensive income (loss), net of tax										20,461	36,385	(69,563)
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications										487,687	254,723	(46,152)
Income tax (expense) benefit										(163,497)	(91,607)	17,239
Change in valuation allowance										161,158	88,833	(20,935)
Total income tax expense, net of valuation allowance		$ (6,584)	$ 695,604	$ (1,322)	$ (3,385)	$ (681)	$ (249)	$ (1,118)	$ (726)	684,313	(2,774)	(3,696)
Net income (loss)		102,418	$ 822,852	$ 113,654	133,076	74,428	$ 72,017	$ 45,522	59,982	1,172,000	251,949	(49,848)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year					1,036,903				744,538	1,036,903	744,538
Balance, end of year		2,236,140				1,036,903				2,236,140	1,036,903	744,538
Accumulated other comprehensive loss
Components of Other Comprehensive Income (Loss) [Abstract]
Other comprehensive income (loss), net of tax										20,461	36,385	(69,563)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year					(81,341)				(117,726)	(81,341)	(117,726)	(48,163)
Other comprehensive income (loss) before reclassifications										33,707	36,399	(67,589)
Less: Amounts reclassified from AOCL										13,246	14	1,974
Balance, end of year		(60,880)				(81,341)				(60,880)	(81,341)	(117,726)
Unrealized gains and losses on available-for- sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year					(57,551)				(148,690)	(57,551)	(148,690)	(25,099)
Other comprehensive income (loss) before reclassifications										51,655	84,223	(121,618)
Less: Amounts reclassified from AOCL										11,252	(6,916)	1,973
Balance, end of year		(17,148)				(57,551)				(17,148)	(57,551)	(148,690)
Defined benefit plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year					(28,938)				23,174	(28,938)	23,174	(44,864)
Other comprehensive income (loss) before reclassifications										(13,720)	(45,182)	68,039
Less: Amounts reclassified from AOCL										1,994	6,930	1
Balance, end of year		(44,652)				(28,938)				(44,652)	(28,938)	23,174
Accumulated foreign currency translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year					$ 5,148				$ 7,790	5,148	7,790	21,800
Other comprehensive income (loss) before reclassifications										(4,228)	(2,642)	(14,010)
Less: Amounts reclassified from AOCL										0	0	0
Balance, end of year		$ 920				$ 5,148				920	5,148	7,790
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications										13,877	114	3,247
Total income tax expense, net of valuation allowance										(631)	(100)	(1,273)
Net income (loss)										13,246	14	1,974
Reclassification from Accumulated Other Comprehensive Income | Unrealized gains and losses on available-for- sale securities
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications	[2]									11,693	(6,816)	3,246
Income tax (expense) benefit										(4,076)	2,402	(924)
Change in valuation allowance	[1]									3,635	(2,502)	(349)
Net income (loss)										11,252	(6,916)	1,973
Reclassification from Accumulated Other Comprehensive Income | Defined benefit plans
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications	[3]									2,184	6,930	1
Income tax (expense) benefit										(764)	(2,425)	0
Change in valuation allowance	[1]									574	2,425	0
Net income (loss)										$ 1,994	$ 6,930	$ 1

[1] See Note 14 – “Income Taxes” for a discussion of the valuation allowance recorded against deferred tax assets.

[2] Increases (decreases) Net realized investment gains on the consolidated statements of operations.

[3] Decreases (increases) Other underwriting and operating expenses, net on the consolidated statements of operations.