Financing arrangements - Summary of gross and net information about repurchase agreements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Gross Amounts of Recognized Liabilities [Member]
Gross And Net Information About Repurchase Agreement Disclosure [Line Items]
Financing arrangements, net	$ 2,595,873	$ 2,972,281
Gross Amounts Offset In The Consolidated Balance Sheets [Member]
Gross And Net Information About Repurchase Agreement Disclosure [Line Items]
Financing arrangements, net	0	0
Net Amounts Of Liabilities Presented In The Consolidated Balance Sheets [Member]
Gross And Net Information About Repurchase Agreement Disclosure [Line Items]
Financing arrangements, net	2,595,873	2,972,281
Gross Amounts Not Offset in the Consolidated Balance Sheets Financial Instruments Posted [Member]
Gross And Net Information About Repurchase Agreement Disclosure [Line Items]
Financing arrangements, net	2,595,873	2,972,281
Gross Amounts Not Offset in the Consolidated Balance Sheets of Cash Collateral Posted [Member]
Gross And Net Information About Repurchase Agreement Disclosure [Line Items]
Financing arrangements, net	0	0
Net Amount [Member]
Gross And Net Information About Repurchase Agreement Disclosure [Line Items]
Financing arrangements, net	$ 0	$ 0