Income Taxes - Components of Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income tax expense from continuing operations	$ (627,615)	$ 232,645	$ 249,894
Shareholders’ equity:
Other comprehensive income (loss)	318,475	186,206	(411,646)
Tax basis compensation expense (from the exercise of stock options and vesting of restricted stock awards) in excess of amounts recognized for financial reporting purposes	0	0	(17,577)
Income tax expense (benefit)	$ (309,140)	$ 418,851	$ (179,329)