Quarterly Financial Data - Condensed Consolidated Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Total assets	$ 393,185	$ 411,045	$ 433,997	$ 403,973	$ 411,975	$ 415,524	$ 446,321	$ 464,153
Total liabilities	93,558	106,029	128,786	103,410	116,112	108,531	114,175	103,178
Total shareholders’ equity	299,627	305,016	305,211	300,563	295,863	306,993	332,146	360,975	356,439
Total liabilities and shareholders’ equity	$ 393,185	$ 411,045	$ 433,997	$ 403,973	$ 411,975	$ 415,524	$ 446,321	$ 464,153