Subsequent Events - Additional Information (Detail) (Subsequent Event [Member], USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended		1 Months Ended
	Jul. 01, 2014 Underwriter [Member]	Jul. 01, 2014 Underwriter [Member]	Jul. 30, 2014 NMGC [Member]	Jul. 30, 2014 NMGI [Member]
Subsequent Event [Line Items]
Number of shares issued through underwriting agreement	15,500,000
Shares offered, price per share		$ 18.10
Additional shares to be offered under underwriting agreement		2,300,000
Number of days option granted to underwriters	30 days
Proceeds from issuance to underwriters		$ 21
Underwriters exercised this option for an additional, shares		1,200,000
Net proceeds from offering through the underwriting	271
Proceeds from issuance of public offering			$ 70	$ 200