Supplemental Information about Changes to Accumulated Other Comprehensive Income - Schedule of Change in Balance by Component of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	$ 4,055,552	$ 4,697,466	$ 3,776,342
Reclassifications, net of tax	10,574	11,517	(14,370)
Ending balance	4,566,861	4,055,552	4,697,466
Total [Member]
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	231,947	997,452	210,981
Other comprehensive income (loss) before reclassifications, net of tax	335,053	(777,022)	800,841
Reclassifications, net of tax	10,574	11,517	(14,370)
Other comprehensive income (loss)	345,627	(765,505)	786,471
Ending balance	577,574	231,947	997,452
Available for Sale Assets
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	332,333	1,090,273	256,196
Other comprehensive income (loss) before reclassifications, net of tax	356,016	(759,976)	855,132
Reclassifications, net of tax	3,965	2,036	(21,055)
Other comprehensive income (loss)	359,981	(757,940)	834,077
Ending balance	692,314	332,333	1,090,273
Deferred Acquisition Costs
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	(5,115)	(10,758)	(6,728)
Other comprehensive income (loss) before reclassifications, net of tax	(1,567)	5,643	(4,030)
Reclassifications, net of tax	0	0	0
Other comprehensive income (loss)	(1,567)	5,643	(4,030)
Ending balance	(6,682)	(5,115)	(10,758)
Foreign Exchange
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	3,627	17,386	24,866
Other comprehensive income (loss) before reclassifications, net of tax	1,340	(13,759)	(7,480)
Reclassifications, net of tax	0	0	0
Other comprehensive income (loss)	1,340	(13,759)	(7,480)
Ending balance	4,967	3,627	17,386
Pension Adjustments
Increase (Decrease) in AOCI, Net of Tax
Beginning balance	(98,898)	(99,449)	(63,353)
Other comprehensive income (loss) before reclassifications, net of tax	(20,736)	(8,930)	(42,781)
Reclassifications, net of tax	6,609	9,481	6,685
Other comprehensive income (loss)	(14,127)	551	(36,096)
Ending balance	$ (113,025)	$ (98,898)	$ (99,449)