Dividends Paid (Detail) - $ / shares	3 Months Ended																12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Capital Stock [Line Items]
Dividends declared per share	$ 0.71	$ 0.34	$ 0.34		$ 0.62	$ 0.28	$ 0.28		$ 0.62	$ 0.28	$ 0.28		$ 0.25	$ 0.2	$ 0	$ 0.53	$ 1.39	$ 1.18	$ 0.98
Dividends per share paid	$ 0.34	$ 0.34	$ 0.34	$ 0.34	$ 0.28	$ 0.28	$ 0.28	$ 0.28	$ 0.28	$ 0.28	$ 0.28	$ 0.28	$ 0.25	$ 0.2	$ 0.2	$ 0.25	$ 1.36	$ 1.12	$ 0.9