Dividends on common stock (Details) (USD $)	0 Months Ended											3 Months Ended											0 Months Ended	3 Months Ended	0 Months Ended
	Mar. 26, 2015	Dec. 12, 2014	Sep. 19, 2014	Jun. 18, 2014	Mar. 27, 2014	Dec. 20, 2013	Sep. 25, 2013	Jun. 18, 2013	Mar. 15, 2013	Dec. 12, 2012	Sep. 12, 2012	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	Apr. 16, 2015	Jun. 30, 2015	Jun. 12, 2015
Dividends on common stock
Capital Program Amount												$ 12,000,000	$ 12,000,000	$ 12,000,000	$ 10,500,000	$ 10,500,000	$ 10,500,000	$ 10,000,000
Dividends declared per common share (in dollars per share)												$ 0.35	$ 0.31	$ 0.30	$ 0.17	$ 0.39	$ 0.37	$ 0.14	$ 0.13	$ 0.12	$ 0.11	$ 0.10
Stock Buyback Amount During the Period												8,119,000	10,555,000	15,943,000	17,888,000	14,196,000
Dividends paid	16,001,000	14,190,000	13,792,000	7,882,000	18,352,000	17,206,000	6,512,000	6,145,000	5,489,000	5,012,000	4,537,000	16,001,000				18,352,000
Payment Under Capital Program Paid As Dividend												1,357,000				10,707,000	10,186,000
Amount Per Share Paid As Dividends Under the Capital Program												$ 0.03				$ 0.23	$ 0.22
Minimum
Dividends on common stock
Capital Program Amount						10,000,000											12,000,000
Subsequent Event
Dividends on common stock
Regular quarterly dividend (in dollars per share)																							0.42
Amount Per Share Paid As Dividends Under the Capital Program																								$ 0.09
Subsequent Event | Expected
Dividends on common stock
Expected value of dividend to be paid																									19,000,000
Payment Under Capital Program Paid As Dividend																								$ 3,900,000