Income Taxes - Principal Reasons for the Difference Between the Effective Tax Rate and UBT Statutory Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Amount [Abstract]
UBT statutory rate	$ 30,956	$ 28,927	$ 25,244
Corporate subsidiaries’ federal, state, local and foreign income taxes	22,162	17,907	31,223
2017 federal tax reform enactment	25,846	0	0
Effect of ASC 740 adjustments, miscellaneous taxes, and other	(5,155)	(1,070)	2,965
Income not taxable resulting from use of UBT business apportionment factors	(20,699)	(17,445)	(14,635)
Income tax expense	$ 53,110	$ 28,319	$ 44,797
Effective Income Tax Rate Reconciliation, Percent [Abstract]
UBT statutory rate, percent	4.00%	4.00%	4.00%
Corporate subsidiaries’ federal, state, local and foreign income taxes, percent	2.90%	2.50%	4.90%
2017 federal tax reform enactment, percent	3.30%	0.00%	0.00%
Effect of ASC 740 adjustments, miscellaneous taxes, and other, percent	(0.70%)	(0.20%)	0.50%
Income not taxable resulting from use of UBT business apportionment factors, percent	(2.60%)	(2.40%)	(2.30%)
Effective tax rate, percent	6.90%	3.90%	7.10%