Investment Securities - (Summary of Held-to-Maturity Securities, Unrealized Losses) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Held-to-maturity Securities [Line Items]
Fair Value, Less than 12 months	$ 0	$ 578,097
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(5,781)
Fair Value, 12 months or more	110,278	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,943)	0
Total Fair Value	110,278	578,097
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1,943)	(5,781)
Residential Mortgage Backed Agency [Member]
Schedule of Held-to-maturity Securities [Line Items]
Fair Value, Less than 12 months	0	472,973
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(1,776)
Fair Value, 12 months or more	35,139	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(72)	0
Total Fair Value	35,139	472,973
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(72)	(1,776)
Other Residential MBS Issued or Guaranteed by U.S. Government Agencies or Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Fair Value, Less than 12 months	0	105,124
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(4,005)
Fair Value, 12 months or more	75,139	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,871)	0
Total Fair Value	75,139	105,124
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (1,871)	$ (4,005)