Notes Payable and Debt and Financing Arrangements - Notes payable and debt (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Commitment, debt		$ 2,675,000	$ 2,675,000
Notes payable and debt		$ 2,648,353	2,647,677
Percentage ownership of subsidiaries		100.00%
Senior notes | 2.3% Senior Notes due 2018
Debt Instrument [Line Items]
Commitment, debt	[1]	$ 300,000	300,000
Notes payable and debt	[2]	$ 298,854	298,686
Debt interest rate (percent)		2.30%
Senior notes | 5.75% Senior Notes due 2021
Debt Instrument [Line Items]
Commitment, debt	[1]	$ 400,000	400,000
Notes payable and debt	[2]	$ 398,061	397,953
Debt interest rate (percent)		5.75%
Senior notes | 6.375% Senior Notes due 2024
Debt Instrument [Line Items]
Commitment, debt	[1]	$ 350,000	350,000
Notes payable and debt	[2]	$ 349,166	349,139
Debt interest rate (percent)		6.375%
Senior notes | 6.25% Senior Notes due 2027
Debt Instrument [Line Items]
Commitment, debt	[1]	$ 325,000	325,000
Notes payable and debt	[2]	$ 323,414	323,375
Debt interest rate (percent)		6.25%
Senior notes | 5.25% Senior Notes due 2043
Debt Instrument [Line Items]
Commitment, debt	[1]	$ 300,000	300,000
Notes payable and debt	[2]	$ 296,460	296,427
Debt interest rate (percent)		5.25%
Subordinated notes | 4.45% Subordinated Notes due 2025
Debt Instrument [Line Items]
Commitment, debt	[1]	$ 500,000	500,000
Notes payable and debt	[2]	$ 493,531	493,329
Debt interest rate (percent)		4.45%
Subordinated notes | 5.5% Subordinated Notes due 2045
Debt Instrument [Line Items]
Commitment, debt	[1]	$ 500,000	500,000
Notes payable and debt	[2]	$ 488,867	$ 488,768
Debt interest rate (percent)		5.50%
Allied International Holdings, Inc. | Trust Preferred Securities Subject to Mandatory Redemption [Member] | Trust Preferred Securities, Face $8 million, due 2035
Debt Instrument [Line Items]
Notes payable and debt		$ 8,200
Allied International Holdings, Inc. | Trust Preferred Securities Subject to Mandatory Redemption [Member] | Trust Preferred Securities, Face $8 million, due 2035 | 3 month LIBOR
Debt Instrument [Line Items]
Spread on variable rate		3.75%

[1] Excluded from the table are certain credit facilities under which the Company is permitted to utilize up to $1.0 billion at March 31, 2017 and December 31, 2016, for revolving loans to support general operating and financing needs. However, at March 31, 2017 and December 31, 2016, $287.7 million and $245 million, respectively, were utilized under these facilities to issue letters of credit, leaving $712.3 million and $755.0 million, respectively, available to support other operating and financing needs.

[2] "In Use/Outstanding" data represent March 31, 2017 and December 31, 2016 accreted values.