Notes Payable and Debt and Financing Arrangements - Financing Structure (Details) € in Thousands, $ in Thousands		Dec. 31, 2017 USD ($)		Sep. 30, 2017 USD ($)	Sep. 30, 2017 EUR (€)	Jun. 30, 2017 EUR (€)	Dec. 31, 2016 USD ($)		Mar. 31, 2015 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2007 USD ($)	Jun. 30, 2007 USD ($)
Debt Instrument [Line Items]
Notes payable and debt	[1]	$ 3,220,769					$ 2,647,677
Senior notes | 2.3% Senior Notes due 2018
Debt Instrument [Line Items]
Notes payable and debt	[1]	$ 299,357					298,686
Debt instrument, face amount										$ 300,000
Debt instrument interest rate, stated percentage		2.30%
Senior notes | 5.75% Senior Notes due 2021
Debt Instrument [Line Items]
Notes payable and debt	[1]	$ 398,384					397,953
Debt instrument, face amount											$ 400,000
Debt instrument interest rate, stated percentage		5.75%
Senior notes | 6.375% Senior Notes due 2024
Debt Instrument [Line Items]
Notes payable and debt	[1]	$ 349,248					349,139
Debt instrument, face amount												$ 350,000
Debt instrument interest rate, stated percentage		6.375%
Senior notes | 5.25% Senior Notes due 2043
Debt Instrument [Line Items]
Notes payable and debt	[1]	$ 323,531					323,375
Debt instrument, face amount													$ 325,000
Debt instrument interest rate, stated percentage		6.25%
Senior notes | 5.25% Senior Notes Due 2043 [Member]
Debt Instrument [Line Items]
Notes payable and debt	[1]	$ 296,560					296,427
Debt instrument, face amount										$ 300,000
Debt instrument interest rate, stated percentage		5.25%
Subordinated debt | 4.45% Subordinated Notes due 2025
Debt Instrument [Line Items]
Notes payable and debt	[1]	$ 494,138					493,329
Debt instrument, face amount									$ 500,000
Debt instrument interest rate, stated percentage		4.45%
Subordinated debt | 5.5% Subordinated Notes due 2045
Debt Instrument [Line Items]
Notes payable and debt	[1],[2]	$ 472,832					488,768
Debt instrument, face amount									$ 500,000
Debt instrument interest rate, stated percentage		5.50%
Subordinated debt | 3.25% Subordinated Notes Due June 2047 [Member]
Debt Instrument [Line Items]
Notes payable and debt		$ 586,719	[1],[3]	$ 558,300			$ 0	[1],[3]
Debt instrument, face amount				$ 568,800	€ 500,000	€ 500,000
Debt instrument interest rate, stated percentage		3.25%		3.25%	3.25%

[1] "Outstanding" data represent December 31, 2017 and December 31, 2016 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 carried 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.