Incentive Plan - Summary of Option Activity (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of Shares, Outstanding at beginning of year	1,751,270	1,740,580	1,732,694
Number of Shares, Granted	238,305	280,375	277,625
Number of Shares, Assumed in acquisition	370,000
Number of Shares, Exercised	(29,480)	(27,634)	(5,147)
Number of Shares, Forfeited	(8,900)	(7,751)	(29,916)
Number of Shares, Expired	(39,264)	(234,300)	(234,676)
Number of Shares, Outstanding at end of year	2,281,931	1,751,270	1,740,580
Number of Shares, Options exercisable at end of year	1,497,960	1,003,075	1,062,786
Weighted Average Exercise Price, Outstanding at beginning of year	$ 15.94	$ 16.47	$ 17.62
Weighted Average Exercise Price, Granted	17.39	17.72	14.68
Weighted Average Exercise Price, Assumed in acquisition	26.11
Weighted Average Exercise Price, Exercised	13.43	12.66	12.63
Weighted Average Exercise Price, Forfeited	14.56	13.30	13.89
Weighted Average Exercise Price, Expired	25.31	22.44	23.28
Weighted Average Exercise Price, Outstanding at end of year	$ 17.62	$ 15.94	$ 16.47