Summary of Characteristics of Interest Rate Swaps (Parenthetical) (Detail) - Forward Starting Pay Fixed Swaps - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Derivative Instruments:
Notional amount	$ 200.0	$ 500.0
Weighted average fixed rate	1.38%	1.44%

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant