Goodwill and other intangibles - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill impairment, mortgage services	$ 35,900	$ 35,749	$ 0	$ 0
Impairment of an intangible asset		1,800	1,700
Other intangibles, gross carrying amount		43,600	43,000
Accumulated amortization and impairment of other intangibles		25,500	16,700
Amortization expense of other intangibles		7,000	6,200	$ 1,900
Future amortization expense
Expected amortization expense in 2016		5,500
Expected amortization expense in 2017		4,600
Expected amortization expense in 2018		3,600
Expected amortization expense in 2019		2,000
Expected amortization expense in 2020		1,000
Title
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill impairment, mortgage services		0
Impairment of an intangible asset		900
Mortgage Services
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill impairment, mortgage services		35,749
Impairment of an intangible asset		$ 900	$ 1,700