Capital Stock - Common Stock (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Aug. 05, 2015
Class of Stock [Line Items]
Stock repurchase program, authorized amount			$ 350,000,000
Number of shares repurchased (shares)	0
Stock repurchase program, remaining authorized repurchase amount	$ 100,000,000
Common dividends declared	$ 94,123,000	$ 184,227,000
Common dividends declared, per share (usd per share)	$ 0.5	$ 0.98
Regular Dividend
Class of Stock [Line Items]
Common dividends declared	$ 94,000,000	$ 90,000,000
Common dividends declared, per share (usd per share)	$ 0.5	$ 0.48
Special Dividend
Class of Stock [Line Items]
Common dividends declared		$ 94,000,000
Common dividends declared, per share (usd per share)		$ 0.50