Subsequent Events (Details) (USD $)	3 Months Ended								12 Months Ended			1 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2012 Loan commitment	Dec. 31, 2012 Bank Borrowings	Mar. 31, 2013 Dividend declared	Feb. 28, 2013 Dividend declared	Jan. 31, 2013 Dividend declared	Feb. 28, 2013 Subsequent Event	Jan. 31, 2013 Subsequent Event	Feb. 28, 2013 Subsequent Event Loan commitment	Jan. 31, 2013 Subsequent Event Loan commitment	Jan. 31, 2013 Subsequent Event Restricted stock $ 36.26 per share	Jan. 31, 2013 Subsequent Event Bank Borrowings
Subsequent Events
Cash dividend per common share	$ 0.465	$ 0.455	$ 0.435	$ 0.435	$ 0.42	$ 0.42	$ 0.42	$ 0.42				$ 0.155	$ 0.155	$ 0.155
Number of shares granted (in shares)																			20,000
Price per share																			$ 36.26
Funded Commitment									$ 134,000						$ 10,600,000	$ 10,600,000
Commitment	5,000,000								5,000,000
Funded loan commitments										2,619,000							128,000	776,000
Loan commitment										10,600,000
Amount borrowed											153,500,000									2,000,000
Description of interest rate											LIBOR									LIBOR
Basis spread over base rate (as a percent)											1.25%									1.25%
Amount outstanding under Unsecured Credit Agreement	115,500,000				56,000,000				115,500,000		115,500,000									117,500,000
Amount available for borrowing under Unsecured Credit Agreement											124,500,000									122,500,000
Debt instrument, amount borrowed																	10,600,000	10,600,000
Remaining loan commitments										7,981,000							7,077,000	7,077,000
Investments funded									18,185,000						488,000	2,484,000
Maximum availability under Unsecured Credit Agreement											$ 240,000,000