Shareholders' Equity - Reconciliation of Changes in Total Shareholders' Equity (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Equity [Abstract]
Beginning balance			$ 975,869,000
Net proceeds from the issuance of common stock			144,111,000
Exercise of stock options			855,000
Earned portion of non-vested stock	$ 1,579,000	$ 1,129,000	3,191,000	$ 2,322,000
Stock option expense	81,000	113,000	117,000	149,000
Deferred compensation - directors			17,000
Adjustment to redemption value on noncontrolling redeemable Operating Partnership units			27,000
Net income attributable to common shareholders	28,532,000	20,576,000	50,983,000	37,249,000	$ 88,531,000	$ 74,126,000	$ 55,128,000
Change in fair value of derivatives	2,421,000	$ (3,726,000)	(1,566,000)	$ (5,655,000)
Dividends			(52,281,000)
Ending balance	$ 1,121,323,000		$ 1,121,323,000		$ 975,869,000