Notes Payable and Debt and Financing Arrangements - Notes payable and debt (Details) € in Thousands, $ in Thousands				6 Months Ended
		Jun. 01, 2018 USD ($)	Jun. 29, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jul. 07, 2017 USD ($)	Jun. 30, 2017 EUR (€)	Jun. 29, 2017 EUR (€)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2007 USD ($)	Jun. 30, 2007 USD ($)
Debt Instrument [Line Items]
Percentage ownership of subsidiaries				100.00%
Notes payable and debt				$ 3,217,123		$ 3,220,769
Proceeds from the issuance of debt				0	$ 558,311
Senior notes | 2.3% Senior Notes due 2018
Debt Instrument [Line Items]
Commitment, debt											$ 300,000
Notes payable and debt	[1]			$ 299,692		299,357
Debt interest rate (percent)				2.30%
Senior notes | 5.75% Senior Notes due 2021
Debt Instrument [Line Items]
Commitment, debt												$ 400,000
Notes payable and debt	[1]			$ 398,600		398,384
Debt interest rate (percent)				5.75%
Senior notes | 6.375% Senior Notes due 2024
Debt Instrument [Line Items]
Commitment, debt													$ 350,000
Notes payable and debt	[1]			$ 349,303		349,248
Debt interest rate (percent)				6.375%
Senior notes | 6.25% Senior Notes due 2027
Debt Instrument [Line Items]
Commitment, debt														$ 325,000
Notes payable and debt	[1]			$ 323,610		323,531
Debt interest rate (percent)				6.25%
Senior notes | 5.25% Senior Notes due 2043
Debt Instrument [Line Items]
Commitment, debt											$ 300,000
Notes payable and debt	[1]			$ 296,626		296,560
Debt interest rate (percent)				5.25%
Subordinated notes | 4.45% Subordinated Notes due 2025
Debt Instrument [Line Items]
Commitment, debt										$ 500,000
Notes payable and debt	[1]			$ 494,542		494,138
Debt interest rate (percent)				4.45%
Subordinated notes | 5.5% Subordinated Notes due 2045
Debt Instrument [Line Items]
Commitment, debt										$ 500,000
Notes payable and debt	[1],[2]			$ 473,024		472,832
Debt interest rate (percent)				5.50%
Debt Instrument, Repurchased Face Amount							$ 16,700
Subordinated notes | 3.25% Subordinated Notes due 2047
Debt Instrument [Line Items]
Commitment, debt			$ 568,800					€ 500,000	€ 500,000
Notes payable and debt	[1],[3]			$ 568,066		586,719
Debt interest rate (percent)			3.25%	3.25%					3.25%
Interest rate term			10 years
Debt Issuance Percentage of Face Value			99.054%						99.054%
Proceeds from the issuance of debt			$ 558,300
Unamortized Debt Issuance Expense			$ 10,500
Subordinated notes | 3.25% Subordinated Notes due 2047 | 3 month Euribor
Debt Instrument [Line Items]
Spread on variable rate			2.90%
Loans Payable [Member]
Debt Instrument [Line Items]
Commitment, debt		$ 14,300
Other debt	[1]			$ 13,660		$ 0
Debt Instrument, Unused Borrowing Capacity, Amount		$ 6,000
Loans Payable [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Spread on variable rate		3.50%

[1] "Outstanding" data represent June 30, 2018 and December 31, 2017 accreted values.

[2] On July 7, 2017, the Company repurchased and canceled $16.7 million of the original debt issuance. See below for further details.

[3] This issuance carries a fixed coupon of 3.25% for a period of ten years, then a floating rate of three-month EURIBOR plus 2.90% from (and including) June 29, 2027 through maturity. The outstanding amount is subject to movement due to foreign exchange.