Stockholders' equity: (Details) (USD $)	0 Months Ended						3 Months Ended						12 Months Ended					0 Months Ended
	Dec. 20, 2013	Sep. 25, 2013	Jun. 18, 2013	Mar. 15, 2013	Dec. 12, 2012	Sep. 12, 2012	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2013 Voting_Right_Per_Share	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Minimum	Dec. 20, 2013 Minimum	Feb. 19, 2014 Subsequent Event	Mar. 07, 2014 Subsequent Event Expected	Mar. 27, 2014 Subsequent Event Expected
Stockholders' equity:
Common stock, shares authorized							75,000,000				75,000,000		75,000,000	75,000,000
Common stock, par value (in dollars per share)							$ 0.001				$ 0.001		$ 0.001	$ 0.001
Preferred stock, authorized but unissued shares (in shares)							10,000						10,000
Preferred stock, par value (in dollars per share)							$ 0.001						$ 0.001
Voting rights per common share													1
Common stock buybacks
Authorized amount of common stock repurchases under the Buyback Program													$ 50,000,000
Remaining authorized amount of common stock repurchases under the Buyback Program													45,800,000
Repurchase of common stock (in shares)													0	100,000	200,000
Repurchase of common stock, amount														1,265,000	2,969,000
Dividends on common stock
Approved quarterly payments under Capital Program																10,500,000	10,000,000
Quarterly payment under return of capital program	10,000,000
Dividend per common share under Capital Program (in dollars per share)	$ 0.22																	$ 0.23
Dividends paid	17,206,000	6,512,000	6,145,000	5,489,000	5,012,000	4,537,000							35,352,000	9,549,000
Amount per Common Share (in dollars per share)	$ 0.37						$ 0.37	$ 0.14	$ 0.13	$ 0.12	$ 0.11	$ 0.10	$ 0.76	$ 0.21					$ 0.39
Regular quarterly dividend (in dollars per share)																		$ 0.16
Dividend payable under Capital Program																		10,500,000
Expected value of dividend to be paid on March 27, 2014																				$ 17,900,000