Stockholders' equity: Dividends on common stock and return of capital program (Details) - USD ($) $ / shares in Units, $ in Thousands												3 Months Ended														12 Months Ended
	Dec. 11, 2015	Nov. 02, 2015	Sep. 11, 2015	Jun. 12, 2015	Mar. 26, 2015	Dec. 20, 2013	Sep. 25, 2013	Jun. 18, 2013	Mar. 15, 2013	Dec. 12, 2012	Sep. 12, 2012	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Dividends on common stock and return of capital program
Suspended quarterly payment		$ 12,000
Dividends declared per common share (in dollars per share)												$ 0.35	$ 0.34	$ 0.42	$ 0.35	$ 0.31	$ 0.30	$ 0.17	$ 0.39	$ 0.37	$ 0.14	$ 0.13	$ 0.12	$ 0.11	$ 0.10	$ 1.46	$ 1.17	$ 0.76
Dividends paid	$ 16,045		$ 15,296	$ 18,972	$ 16,001	$ 17,206	$ 6,512	$ 6,145	$ 5,489	$ 5,012	$ 4,537					$ 14,190	$ 13,792	$ 7,882	$ 18,352							$ 66,314	$ 54,216	$ 35,352
Capital Program Amount													$ 12,000	$ 12,000	$ 12,000	12,000	12,000	10,500	10,500	$ 10,500	$ 10,000						12,000	10,500
Stock Buyback Amount During the Period													$ 12,169	19,106	8,119	$ 10,555	$ 15,943	$ 17,888	14,196							39,400	$ 58,600
Payment Under Capital Program Paid As Dividend														$ 4,019	$ 1,357				$ 10,707	$ 10,186
Amount Per Share Paid As Dividends Under the Capital Program														$ 0.09	$ 0.03				$ 0.23	$ 0.22
Minimum
Dividends on common stock and return of capital program
Quarterly payment under return of capital program																										$ 12,000		$ 10,000