Borrowings (Interest Rate Ranges, Maturity and Balance of Indebtedness) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
Debt Instrument [Line Items]
Total outstanding debt	$ 3,472,718		$ 1,673,800
Unsecured Debt
Debt Instrument [Line Items]
Total outstanding debt	1,681,783
Fixed Rate Debt
Debt Instrument [Line Items]
Total outstanding debt	3,062,600		1,291,400
Fixed Rate Debt | Taxable Secured
Debt Instrument [Line Items]
Actual Interest Rates, Minimum	3.51%
Actual Interest Rates, Maximum	6.21%
Average Interest Rates	4.07%
Total outstanding debt	1,161,800		422,400
Fixed Rate Debt | Unsecured Debt
Debt Instrument [Line Items]
Actual Interest Rates, Minimum	3.15%
Actual Interest Rates, Maximum	5.57%
Average Interest Rates	5.02%
Total outstanding debt	1,131,800		310,000
Fixed Rate Debt | Interest Rate Swap
Debt Instrument [Line Items]
Actual Interest Rates, Minimum	4.38%
Actual Interest Rates, Maximum	6.63%
Average Interest Rates	4.76%
Total outstanding debt	769,000		559,000
Fixed Rate Debt | Minimum | Taxable Secured
Debt Instrument [Line Items]
Maturity date	2014
Fixed Rate Debt | Minimum | Unsecured Debt
Debt Instrument [Line Items]
Maturity date	2017
Fixed Rate Debt | Minimum | Interest Rate Swap
Debt Instrument [Line Items]
Maturity date	2013
Fixed Rate Debt | Maximum | Taxable Secured
Debt Instrument [Line Items]
Maturity date	2035
Fixed Rate Debt | Maximum | Unsecured Debt
Debt Instrument [Line Items]
Maturity date	2024
Fixed Rate Debt | Maximum | Interest Rate Swap
Debt Instrument [Line Items]
Maturity date	2017
Variable Rate Debt
Debt Instrument [Line Items]
Total outstanding debt	410,100		382,400
Variable Rate Debt | Taxable Secured
Debt Instrument [Line Items]
Actual Interest Rates, Minimum	0.66%	[1]
Actual Interest Rates, Maximum	1.13%	[1]
Average Interest Rates	0.72%	[1]
Total outstanding debt	122,700	[1]	70,900	[1]
Variable Rate Debt | Unsecured Debt
Debt Instrument [Line Items]
Average Interest Rates	0.00%	[1]
Maturity date	2015	[1]
Total outstanding debt	0	[1]	23,000	[1]
Interest rate, stated percentage	1.46%	[1]
Variable Rate Debt | Interest Rate Cap
Debt Instrument [Line Items]
Actual Interest Rates, Minimum	0.72%	[1]
Actual Interest Rates, Maximum	1.06%	[1]
Average Interest Rates	0.80%	[1]
Total outstanding debt	$ 287,400	[1]	$ 288,500	[1]
Variable Rate Debt | Minimum | Taxable Secured
Debt Instrument [Line Items]
Maturity date	2013	[1]
Variable Rate Debt | Minimum | Interest Rate Cap
Debt Instrument [Line Items]
Maturity date	2013	[1]
Variable Rate Debt | Maximum | Taxable Secured
Debt Instrument [Line Items]
Maturity date	2033	[1]
Variable Rate Debt | Maximum | Interest Rate Cap
Debt Instrument [Line Items]
Maturity date	2033	[1]

[1] Amounts are adjusted to reflect interest rate swap and cap agreements in effect at December 31, 2013, and 2012, respectively, which results in the Company paying fixed interest payments over the terms of the interest rate swaps and on changes in interest rates above the strike rate of the cap.