Parent Company Only Financial Information, Statement of Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
INCOME
Interest income on notes receivable from subsidiaries	$ 749,000	$ 659,538	$ 615,627
Total income	1,073,900	1,060,156	1,005,635
EXPENSE
Income before income tax expense (benefit) and equity in undistributed net income (loss) of subsidiaries	338,767	287,596	269,788
Income tax expense	109,503	87,322	81,487
Net income	229,264	200,274	188,301
Preferred stock dividends	9,347	8,903	7,155
Net income available to common equity	219,917	191,371	181,146
Parent Company Only
INCOME
Dividends from subsidiaries	213,000	188,000	205,000
Interest income on notes receivable from subsidiaries	4,175	0	0
Other income	1,763	4,669	8,441
Total income	218,938	192,669	213,441
EXPENSE
Interest expense on short and long-term funding	18,464	21,901	39,576
Other expense	6,927	4,289	4,684
Total expense	25,391	26,190	44,260
Income before income tax expense (benefit) and equity in undistributed net income (loss) of subsidiaries	193,547	166,479	169,181
Income tax expense	4,768	3,468	(1,665)
Income before equity in undistributed net income (loss) of subsidiaries	188,779	163,011	170,846
Equity in undistributed net income (loss) of subsidiaries	40,485	37,263	17,455
Net income	229,264	200,274	188,301
Preferred stock dividends	9,347	8,903	7,155
Net income available to common equity	$ 219,917	$ 191,371	$ 181,146