Common stock buyback programs: (Details) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014
Common stock buyback program:
Remaining authorized amount of common stock repurchases under the Buyback Program	$ 29,100,000
Repurchase of common stock (in shares)	0.2				0.4
Cost of shares of common stock repurchase under buyback program	$ 8,119,000	$ 10,555,000	$ 15,943,000	$ 17,888,000	$ 14,196,000