Equity method investment Equity method investment - Financial Position and Results of Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Assets	$ 34,932,860	$ 27,805,496
Liabilities	30,028,249	23,449,748
Total liabilities and stockholders' equity	34,932,860	27,805,496
Bankers Healthcare Group, LLC
Schedule of Equity Method Investments [Line Items]
Assets	1,330,317	840,398
Liabilities	1,088,135	641,037
Equity interests	242,182	199,361
Total liabilities and stockholders' equity	1,330,317	840,398
Bankers Healthcare Group, LLC
Schedule of Equity Method Investments [Line Items]
Revenues	457,928	366,500	$ 220,253
Income before income taxes and equity in undistributed income of subsidiaries	$ 171,964	$ 182,462	$ 104,297