Capital Stock - Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income available to common shareholders	$ 98,208	$ 129,832	$ 105,617	$ 157,524	$ 219,454	$ 172,817	$ 74,127	$ 83,098	$ 491,184	$ 549,494	$ 250,349
Effect of dilutive securities:									0	0	0
Dilutive net income available to common shareholders									$ 491,184	$ 549,494	$ 250,349
Denominator:
Weighted average basic shares (shares)									187,780,355	187,728,634	199,563,196
Effect of dilutive securities (shares)									506,965	296,204	86,981
Weighted average dilutive shares (shares)									188,287,320	188,024,838	199,650,177
Net income per average share attributable to common stockholders - Basic (usd per share)	$ 0.52	$ 0.69	$ 0.56	$ 0.84	$ 1.17	$ 0.92	$ 0.39	$ 0.44	$ 2.62	$ 2.93	$ 1.25
Net income per average share attributable to common stockholders - Diluted (usd per share)									$ 2.61	$ 2.92	$ 1.25