Short-Term Borrowings and Long-Term Debt - Interest Expense and Senior Notes (Details) - USD ($)			1 Months Ended	12 Months Ended
	Dec. 21, 2017	Jan. 31, 2015	Sep. 30, 2010	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 15, 2007	Oct. 31, 2003
Debt Disclosure [Abstract]
Interest expense on debt				$ 46,600,000	$ 36,100,000	$ 37,300,000
Weighted average interest rate on short-term borrowings				2.62%	1.39%
Debt Instrument [Line Items]
Long-term debt				$ 696,465,000	$ 695,492,000
3.50% Senior Notes
Debt Instrument [Line Items]
Principal value		$ 350,000,000
Proceeds from issuance of senior notes, net of discount and issuance cost		$ 346,400,000
Debt issuance costs				2,200,000
Discount on debt				200,000
5.375% Senior Notes
Debt Instrument [Line Items]
Principal value			$ 350,000,000
Proceeds from issuance of senior notes, net of discount and issuance cost			345,000,000
Payments for settlement of 3.875% Convertible Notes			$ 250,000,000
6.05% Subordinated Notes
Debt Instrument [Line Items]
Principal value							$ 250,000,000
7.0% Junior Subordinated Debentures
Debt Instrument [Line Items]
Principal value								$ 50,000,000.0
Aggregate principal amount of debenture redeemed	$ 51,500,000
Senior Notes | 3.50% Senior Notes
Debt Instrument [Line Items]
Principal value				350,000,000
Long-term debt				347,639,000	347,303,000
Senior Notes | 5.375% Senior Notes
Debt Instrument [Line Items]
Principal value				350,000,000
Long-term debt				$ 348,826,000	$ 348,189,000