Incentive Plan - Summary of Option Activity (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of Shares, Outstanding at beginning of year	1,740,580	1,732,694	2,043,933
Number of Shares, Granted	280,375	277,625	270,250
Number of Shares, Exercised	(27,634)	(5,147)	(36,218)
Number of Shares, Forfeited	(234,300)	(29,916)	(202,928)
Number of Shares, Expired	(7,751)	(234,676)	(342,343)
Number of Shares, Outstanding at end of year	1,751,270	1,740,580	1,732,694
Number of Shares, Options exercisable at end of year	1,003,075	1,062,786	1,113,185
Weighted Average Exercise Price, Outstanding at beginning of year	$ 16.47	$ 17.62	$ 18.21
Weighted Average Exercise Price, Granted	$ 17.72	$ 14.68	$ 13.83
Weighted Average Exercise Price, Exercised	$ 12.66	$ 12.63	$ 11.55
Weighted Average Exercise Price, Forfeited	$ 22.44	$ 13.89	$ 18.28
Weighted Average Exercise Price, Expired	$ 13.30	$ 23.28	$ 17.92
Weighted Average Exercise Price, Outstanding at end of year	$ 15.94	$ 16.47	$ 17.62