Unsecured Debt and Lines of Credit Secured Note Summary (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Apr. 01, 2014
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 1,900,000	$ 600,000
Unsecured Bonds 5.500% [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	300,000
Unsecured Bonds 5.200% [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	300,000
Unsecured Bonds 3.625% [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	300,000	300,000
Unsecured Bonds 3.375% [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	300,000
Unsecured Bonds 3.25% [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	300,000	300,000
Unsecured Bonds 3.875% [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	400,000
Senior Notes [Member] | Unsecured Bonds 5.500% [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.50%
Senior Notes [Member] | Unsecured Bonds 5.200% [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.20%
Senior Notes [Member] | Unsecured Bonds 3.625% [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.625%
Senior Notes [Member] | Unsecured Bonds 3.375% [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.375%
Senior Notes [Member] | Unsecured Bonds 3.25% [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.25%
Senior Notes [Member] | Unsecured Bonds 3.875% [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount			$ 400,000
Debt Instrument, Interest Rate, Stated Percentage	3.875%		3.875%