Debt (Narrative) (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 27, 2014	Dec. 31, 2013
Proceeds from convertible senior notes	$ 200,000,000	$ 0
Line of Credit Facility, Maximum Borrowing Capacity	700,000,000
Long-term Debt	79,780,000			80,080,000
Debt Instrument, Convertible, Conversion Ratio	13.926
Convertible Debt Instrument, Conversion Ratio, Principal Amount, Basis	1,000
Debt Instrument, Convertible, Conversion Price	$ 71.81
Debt Instrument, Convertible, Number of Equity Instruments	2,785,200
Convertible Debt	192,253,000		192,238,000	0
Debt Instrument, Convertible, Carrying Amount of Equity Component	7,762,000
Credit Facility, Current Revolving Borrowing Capacity	450,000,000
Credit Facility Expansion Amount	130,000,000
Debt Instrument, Unused Borrowing Capacity, Fee, Basis Points	40
Debt Instrument, Name, Convertible Senior Notes [Member]
Debt Instrument, Interest Rate, Stated Percentage	3.25%
Debt Issuance Cost	5,969,000
Debt Instrument, Name, Revolving Credit Facility [Member]
Debt Maturity, Extension Option, Number Of Years	1
Number of basis points over LIBOR	150
Unused balance of the unsecured revolving credit facility				343,000,000
Debt Instrument Name, Old Revolving Credit Facility [Member]
Line of Credit Facility, Maximum Borrowing Capacity	620,000,000
Debt Instrument Name, $130m Term Loan - June 2020 Maturity [Member]
Term Loans, Outstanding Balance	130,000,000
Number of basis points over LIBOR	175
Debt Instrument Name, $120m Term Loans - June 2020 Maturity [Member]
Term Loans, Outstanding Balance	120,000,000
Number of basis points over LIBOR	175
Debt Instrument, Name, Fannie Mae (A) [Member]
Long-term Debt				70,840,000
Debt Instrument, Interest Rate, Stated Percentage				6.85%
Debt Instrument, Name, Fannie Mae (B) [Member]
Long-term Debt				7,208,000
Debt Instrument, Interest Rate, Stated Percentage				7.17%
Interest Rate Swap, Date Entered, March 2014 [Member]
Derivative, Fixed Interest Rate	3.91%
Derivative, Notional Amount	130,000,000
Interest Rate Swap, Date Entered, May 2012 [Member]
Derivative, Fixed Interest Rate	3.29%
Derivative, Notional Amount	40,000,000
Interest Rate Swap, Date Entered, June 2013 [Member]
Derivative, Fixed Interest Rate	3.86%
Derivative, Notional Amount	80,000,000
Allocated to Debt [Member] | Debt Instrument, Name, Convertible Senior Notes [Member]
Debt Issuance Cost	5,706,000
Allocated to Equity [Member] | Debt Instrument, Name, Convertible Senior Notes [Member]
Debt Issuance Cost	$ 263,000