Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Mar. 31, 2016
Schedule Of Accounting Policies [Line Items]
Maturity period of cash and cash equivalents	Three months or less
Scenario, Forecast [Member]
Schedule Of Accounting Policies [Line Items]
Unamortized deferred debt issuance costs		$ 3.1

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_UnamortizedDebtIssuanceExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant