Debt (Details) - USD ($)		9 Months Ended	12 Months Ended
		Sep. 30, 2017	Dec. 31, 2016	Apr. 30, 2015	Jun. 30, 2013
Debt [Line Items]
Notes payable and debt		$ 225,423,000	$ 125,297,000
Long-term debt		1,732,367,000	1,701,966,000
Total debt		1,957,790,000	1,827,263,000
Line of Credit [Line Items]
Foreign subsidiary lines of credit		423,000	297,000
Line of credit maximum borrowing capacity		$ 90,000,000	$ 79,000,000
Line of credit interest rate during the period		1.48%	1.49%
Long-Term Liabilities
Debt [Line Items]
Unamortized debt issuance costs		$ (2,633,000)	$ (3,034,000)
Senior unsecured notes - Series B [Member]
Debt [Line Items]
Stated interest rate on debt instrument		5.00%
Long-term debt		$ 100,000,000	100,000,000
Senior unsecured notes - Series D [Member]
Debt [Line Items]
Stated interest rate on debt instrument		3.22%
Notes payable and debt		$ 100,000,000
Long-term debt			100,000,000
Senior unsecured notes - Series E [Member]
Debt [Line Items]
Stated interest rate on debt instrument		3.97%
Long-term debt		$ 50,000,000	50,000,000
Senior unsecured notes - Series F [Member]
Debt [Line Items]
Stated interest rate on debt instrument		3.40%
Long-term debt		$ 100,000,000	100,000,000
Senior unsecured notes - Series G [Member]
Debt [Line Items]
Stated interest rate on debt instrument		3.92%
Long-term debt		$ 50,000,000	50,000,000
Senior unsecured notes - Series H [Member]
Debt [Line Items]
Interest rate terms on debt		* Series H senior unsecured notes bear interest at a 3-month LIBOR for that floating rate interest period plus 1.25%.
Long-term debt	[1]	$ 50,000,000	50,000,000
Senior unsecured notes - Series I [Member]
Debt [Line Items]
Stated interest rate on debt instrument		3.13%
Long-term debt		$ 50,000,000	50,000,000
Senior unsecured notes - Series J [Member]
Debt [Line Items]
Interest rate terms on debt		** Series J senior unsecured notes bear interest at a 3-month LIBOR for that floating rate interest period plus 1.45%.
Long-term debt	[2]	$ 40,000,000	40,000,000
Senior unsecured notes - Series K [Member]
Debt [Line Items]
Stated interest rate on debt instrument		3.44%
Long-term debt		$ 160,000,000	160,000,000
Credit Agreements
Debt [Line Items]
Interest rate terms on debt		The interest rates applicable to the Amended Credit Agreement are, at the Company’s option, equal to either the alternate base rate calculated daily (which is a rate per annum equal to the greatest of (a) the prime rate in effect on such day, (b) the federal funds effective rate in effect on such day plus 1/2% per annum, or (c) the adjusted LIBO rate on such day (or if such day is not a business day, the immediately preceding business day) for a deposit in U.S. dollars with a maturity of one month plus 1% per annum) or the applicable 1, 2, 3 or 6 month adjusted LIBO rate, in each case, plus an interest rate margin based upon the Company’s leverage ratio, which can range between 0 basis points to 12.5 basis points for alternate base rate loans and between 80 basis points and 117.5 basis points for adjusted LIBO rate loans.
Debt facility fee		The facility fee on the Amended Credit Agreement ranges between 7.5 basis points and 20 basis points.
Debt covenant description		The Amended Credit Agreement requires that the Company comply with an interest coverage ratio test of not less than 3.50:1 as of the end of any fiscal quarter for any period of four consecutive fiscal quarters and a leverage ratio test of not more than 3.50:1 as of the end of any fiscal quarter.
Notes payable and debt		$ 125,000,000	125,000,000
Long-term debt		1,135,000,000	1,005,000,000
Unused borrowing capacity		338,000,000	468,000,000
Credit Agreements | Term loan facility [Member]
Debt [Line Items]
Face value of debt					$ 300,000,000
Credit Agreements | Revolving facilities [Member]
Debt [Line Items]
Face value of debt				$ 1,300,000,000	$ 1,100,000,000
Notes payable and debt		125,000,000
Long-term debt		$ 835,000,000
Unsecured debt
Debt [Line Items]
Call feature on debt instrument		The Company may prepay all or some of the senior unsecured notes at any time in an amount not less than 10% of the aggregate principal amount outstanding, plus the applicable make-whole amount or prepayment premium for Series H and J senior unsecured notes. In the event of a change in control of the Company (as defined in the note purchase agreement), the Company may be required to prepay the senior unsecured notes at a price equal to 100% of the principal amount thereof, plus accrued and unpaid interest.
Debt covenant description		These senior unsecured notes require that the Company comply with an interest coverage ratio test of not less than 3.50:1 for any period of four consecutive fiscal quarters and a leverage ratio test of not more than 3.50:1 as of the end of any fiscal quarter.
Total debt		$ 700,000,000	$ 700,000,000
Credit Agreements and Unsecured Debt [Member]
Debt [Line Items]
Weighted-average interest rate		2.82%	2.55%

[1] * Series H senior unsecured notes bear interest at a 3-month LIBOR for that floating rate interest period plus 1.25%.

[2] ** Series J senior unsecured notes bear interest at a 3-month LIBOR for that floating rate interest period plus 1.45%.