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, 2015	Dec. 31, 2014	Dec. 31, 2013
Effective Income Tax Rate Reconciliation, Amount [Abstract]
UBT statutory rate	$ 25,244	$ 24,345	$ 22,570
Corporate subsidiaries’ federal, state, local and foreign income taxes	25,720	24,516	27,766
Effect of ASC 740 adjustments, miscellaneous taxes, and other	2,643	2,586	(687)
Income not taxable resulting from use of UBT business apportionment factors	(15,485)	(13,665)	(12,820)
Income tax expense	$ 38,122	$ 37,782	$ 36,829
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	4.10%	4.00%	4.90%
Effect of ASC 740 adjustments, miscellaneous taxes, and other, percent	0.40%	0.40%	(0.10%)
Income not taxable resulting from use of UBT business apportionment factors, percent	(2.50%)	(2.20%)	(2.30%)
Effective tax rate, percent	6.00%	6.20%	6.50%