Debt (Narrative) (Details) - USD ($)				12 Months Ended
	Sep. 28, 2015	Apr. 11, 2015	Mar. 16, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Nov. 14, 2014
Debt Instrument [Line Items]
Maximum amount of borrowings outstanding				$ 324,000,000	$ 79,500,000	$ 76,000,000
Weighted average amount of borrowings outstanding				$ 109,700,000	$ 12,500,000	$ 10,500,000
Weighted average interest rate before amortization of debt fees				1.10%	1.10%	1.30%
Annual facility fee				$ 900,000
Revolving credit facility, max capacity				600,000,000
Issuance of senior notes, net of costs				456,151,000	$ 244,579,000	$ 564,389,000
Senior Notes
Debt Instrument [Line Items]
Principal balance				1,744,200,000	1,494,200,000
4.50% notes | Senior Notes
Debt Instrument [Line Items]
Principal balance			$ 200,000,000	$ 450,000,000	250,000,000		$ 250,000,000
Interest rate			4.50%	4.50%
Note Offering Percent			105.38%
Debt yield to maturity percent			4.18%
Issuance of senior notes, net of costs			$ 208,600,000
6.20% Senior notes | Senior Notes
Debt Instrument [Line Items]
Early Repayment of Senior Debt		$ 222,200,000
Prepayment premium on senior note		19,200,000
Accrued but unpaid interest		$ 3,000,000
Interest rate		6.20%
Repayments of Debt		$ 200,000,000
2.55% notes | Senior Notes
Debt Instrument [Line Items]
Principal balance	$ 250,000,000			$ 250,000,000	$ 0
Interest rate	2.55%			2.55%
Issuance of senior notes, net of costs	$ 247,500,000