Capital Stock - Common Stock (Detail) - USD ($)	3 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Aug. 05, 2015
Class of Stock [Line Items]
Stock repurchase program, authorized amount	$ 350,000,000					$ 250,000,000
Stock repurchase program, additional authorized amount	$ 100,000,000
Number of shares repurchased (shares)		18,000,000	0
Repurchase price per share (usd per share)		$ 13.94
Value of shares repurchased		$ 250,000,000
Stock repurchase program, remaining authorized repurchase amount			$ 100,000,000
Common dividends declared			$ 458,735,000	$ 378,804,000	$ 369,855,000
Common dividends declared, per share (usd per share)			$ 2.44	$ 1.92	$ 1.80
Regular Dividend
Class of Stock [Line Items]
Common dividends declared			$ 364,000,000	$ 379,000,000	$ 370,000,000
Common dividends declared, per share (usd per share)			$ 1.94	$ 1.92	$ 1.80
Special Dividend
Class of Stock [Line Items]
Common dividends declared			$ 94,000,000
Common dividends declared, per share (usd per share)			$ 0.50