Capital Stock - Issuances of Common Stock (Details)	6 Months Ended
	Jun. 30, 2017 USD ($) $ / shares shares
Stock Issued During Period, Value, New Issues	$ 122,857,000
At the Market Offering Program [Member] | 2017-Q1 [Member]
Share Price | $ / shares	$ 10.13
Stock Issued During Period Shares New Issues | shares	1,286,196
Stock Issued During Period, Value, New Issues	$ 12,792,000
At the Market Offering Program [Member] | 2017-Q2 [Member]
Share Price | $ / shares	$ 10.17
Stock Issued During Period Shares New Issues | shares	11,012,836
Stock Issued During Period, Value, New Issues	$ 110,065,000
At the Market Offering Program [Member] | 2017-Total[Member]
Stock Issued During Period Shares New Issues | shares	12,299,032
Stock Issued During Period, Value, New Issues	$ 122,857,000
At the Market Offering Program [Member] | 2016-Q2 [Member]
Share Price | $ / shares	$ 10.48
Stock Issued During Period Shares New Issues | shares	646,753
Stock Issued During Period, Value, New Issues	$ 6,591,000
At the Market Offering Program [Member] | 2016-Q3 [Member]
Share Price | $ / shares	$ 10.8
Stock Issued During Period Shares New Issues | shares	3,818,802
Stock Issued During Period, Value, New Issues	$ 40,525,000
At the Market Offering Program [Member] | 2016-Q4 [Member]
Share Price | $ / shares	$ 10.79
Stock Issued During Period Shares New Issues | shares	6,707,101
Stock Issued During Period, Value, New Issues	$ 71,212,000
At the Market Offering Program [Member] | 2016-Total [Member]
Stock Issued During Period Shares New Issues | shares	11,172,656
Stock Issued During Period, Value, New Issues	$ 118,328,000