Earnings per Share - Composition of Weighted-Average Common Shares (Denominator) Used in Basic and Diluted Earnings Per Share (Detail) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Earnings Per Share [Abstract]
Weighted Average Number of Shares Issued, Basic	105,563,455	105,563,455	105,563,455	105,563,455
Average treasury shares (in shares)	(16,620,341)	(16,445,760)	(16,621,717)	(15,480,232)
Average unearned nonvested shares	(111,356)	(195,303)	(105,815)	(189,561)
Weighted average common shares and common stock equivalents used to calculate basic earnings per share (in shares)	88,831,758	88,922,392	88,835,923	89,893,662
Additional common stock equivalents (nonvested stock) used to calculate diluted earnings per share	6,856	16,611	4,760	9,888
Weighted average common shares and common stock equivalents used to calculate diluted earnings per share (in shares)	88,838,614	88,939,003	88,840,683	89,903,550