Segments - Additional Information (Detail) $ in Millions	3 Months Ended
Mar. 31, 2017 USD ($) Segment	Mar. 31, 2016 USD ($)
Segment Reporting [Abstract]
Number of reportable segments \| Segment	2
Interest income \| $	$ 2.2	$ 1.6

Amount before accretion (amortization) of purchase discount (premium) of interest income on nonoperating securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7(b))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration