Fair Value Measurements - Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financial assets:
Trading securities	$ 676,411	$ 265,534
Available for sale securities	765,542	598,007
Financial liabilities:
Securities sold, not yet purchased, at fair value	173,509	153,889
Recurring
Financial assets:
Trading securities	676,411	265,534
Available for sale securities	765,542	598,007
Loans held for sale	1,859,820	1,748,498
Derivative assets	55,563	57,036
Mortgage servicing asset	47,766	61,968
Financial liabilities:
Securities sold, not yet purchased, at fair value	173,509	153,889
Derivative liabilities	17,500	35,737
Recurring | Level 1
Financial assets:
Trading securities	3,526	9,481
Available for sale securities	20,983	19,840
Financial liabilities:
Securities sold, not yet purchased, at fair value	85,471	60,715
Recurring | Level 2
Financial assets:
Trading securities	672,885	256,053
Available for sale securities	744,559	578,167
Loans held for sale	1,828,275	1,712,697
Derivative assets	55,563	57,036
Financial liabilities:
Securities sold, not yet purchased, at fair value	88,038	93,174
Derivative liabilities	17,500	35,737
Recurring | Level 3
Financial assets:
Loans held for sale	31,545	35,801
Mortgage servicing asset	$ 47,766	$ 61,968