Derivatives and Fair Value Measurements - Paid to and Received from Counterparties for Settlements of Matured Swaps (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Paid to counterparties	$ 1,216	$ 3,931	$ 5,620
Received from counterparties	$ 6	$ 6	$ 6

The cash inflow provided by derivative instruments during the period, which are classified as investing activities, excluding those designated as hedging instruments.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3179-108585

Name:	us-gaap_ProceedsFromDerivativeInstrumentInvestingActivities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration