Goodwill and other intangibles - Additional Information (Detail) - USD ($)		12 Months Ended
	Feb. 10, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Acquired Finite-Lived Intangible Assets [Line Items]
Impairment of goodwill	$ 35,900,000	$ 0	$ 35,749,000	$ 0
Impairment of an intangible asset		0	1,800,000	1,700,000
Other intangibles, gross carrying amount		32,700,000	43,600,000
Accumulated amortization and impairment of other intangibles		21,800,000	25,500,000
Amortization expense of other intangibles		5,500,000	7,000,000
Future amortization expense
Expected amortization expense in 2017		3,400,000
Expected amortization expense in 2018		2,800,000
Expected amortization expense in 2019		2,000,000
Expected amortization expense in 2020		1,000,000
Expected amortization expense in 2021		400,000
Ancillary Services and Corporate
Acquired Finite-Lived Intangible Assets [Line Items]
Impairment of goodwill		0	34,180,000	0
Impairment of an intangible asset			900,000
Other intangibles, gross carrying amount		11,200,000
Accumulated amortization and impairment of other intangibles		9,100,000
Title
Acquired Finite-Lived Intangible Assets [Line Items]
Impairment of goodwill		$ 0	1,569,000	$ 0
Impairment of an intangible asset			$ 900,000