Restricted Stock, Performance Share and Cash Awards - Schedule of Restricted Stock Awards Vesting Periods (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Vesting Period [Line Items]
Shares granted in the period	376,541	369,975	361,400
Vesting Period One Year [Member]
Vesting Period [Line Items]
Shares granted in the period	19,250	19,375	20,000
Vesting period, years	1 year
Vesting Period Three Years [Member]
Vesting Period [Line Items]
Shares granted in the period	33,741
Vesting period, years	3 years
Vesting Period Four Years [Member]
Vesting Period [Line Items]
Shares granted in the period	323,550	345,000	332,000
Vesting period, years	4 years
Vesting Period Five Years [Member]
Vesting Period [Line Items]
Shares granted in the period		5,600	9,400
Vesting period, years	5 years