Fair Values of Financial Instruments - Additional Information (Detail)	6 Months Ended	12 Months Ended
Jun. 30, 2017	Dec. 31, 2016
4.97% Series A Senior Notes, Due October 5th 2020
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Current market rates used in measuring fair value	4.59%	6.65%
4.27% Series B Senior Notes, Due June 14th 2022
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Current market rates used in measuring fair value	4.88%	7.02%

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration