Dividends (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
December 6, 2010 [Member]
Quarterly dividends declared by board of directors
Dividend per Common Share	$ 0.100
Aggregate Dividend Amount	$ 7,964
Record Date	Dec. 31, 2010
Payment Date	Jan. 14, 2011
March 8, 2011 [Member]
Quarterly dividends declared by board of directors
Dividend per Common Share	$ 0.100
Aggregate Dividend Amount	7,857
Record Date	Mar. 31, 2011
Payment Date	Apr. 15, 2011
June 27, 2011 [Member]
Quarterly dividends declared by board of directors
Dividend per Common Share	$ 0.125
Aggregate Dividend Amount	9,364
Record Date	Jul. 07, 2011
Payment Date	Jul. 15, 2011
September 14, 2011 [Member]
Quarterly dividends declared by board of directors
Dividend per Common Share	$ 0.125
Aggregate Dividend Amount	9,035
Record Date	Sep. 30, 2011
Payment Date	Oct. 14, 2011
November 7, 2011 [Member]
Quarterly dividends declared by board of directors
Dividend per Common Share	$ 0.150
Aggregate Dividend Amount	10,839
Record Date	Nov. 30, 2011
Payment Date	Dec. 15, 2011
February 17, 2012 [Member]
Quarterly dividends declared by board of directors
Dividend per Common Share	$ 0.150
Aggregate Dividend Amount	10,865
Record Date	Feb. 29, 2012
Payment Date	Mar. 15, 2012
May 2, 2012 [Member]
Quarterly dividends declared by board of directors
Dividend per Common Share	$ 0.150
Aggregate Dividend Amount	10,847
Record Date	May 31, 2012
Payment Date	Jun. 15, 2012
August 1, 2012 [Member]
Quarterly dividends declared by board of directors
Dividend per Common Share	$ 0.150
Aggregate Dividend Amount	$ 10,464
Record Date	Aug. 31, 2012
Payment Date	Sep. 14, 2012