Notes Payable and Debt and Financing Arrangements - Notes payable and debt (Details) € in Thousands, $ in Thousands			3 Months Ended
		Jun. 29, 2017 USD ($)	Mar. 31, 2018 USD ($)	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			$ 3,240,461	$ 3,220,769
Senior notes | 2.3% Senior Notes due 2018
Debt Instrument [Line Items]
Commitment, debt									$ 300,000
Notes payable and debt	[1]		$ 299,525	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]		$ 398,492	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]		$ 349,275	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]		$ 323,571	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]		$ 296,593	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]		$ 494,340	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]		$ 472,928	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]		$ 605,737	$ 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 Issuance of Long-term 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%

[1] "Outstanding" data represent March 31, 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.